As also mentioned on https://wiki.jenkins.io/display/JENKINS/Gitlab+Merge+Request+Builder+Plugin?focusedCommentId=143494428#comment-143494428

the Gitlab Merge Request Builder plug-In still uses the deprecated API v3. by now it should be v4 !  (https://docs.gitlab.com/ee/api/v3_to_v4.html)

Therefor we get an error while trying to create a Job (and ofcourse it never runs since the connection doesn't work)

java.io.FileNotFoundException: https://our.gitlab.installation/api/v3/projects

→ suggestion: change request URL from <server defined in config>/api/v3/projects to <server defined in config>/api/v4/projects

Full stack trace
java.io.FileNotFoundException: https://holo-git.holo.local/api/v3/projects
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(Unknown Source)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(Unknown Source)
at org.gitlab.api.http.GitlabHTTPRequestor.parse(GitlabHTTPRequestor.java:324)
at org.gitlab.api.http.GitlabHTTPRequestor.access$200(GitlabHTTPRequestor.java:28)
at org.gitlab.api.http.GitlabHTTPRequestor$1.fetch(GitlabHTTPRequestor.java:232)
Caused: java.lang.Error
at org.gitlab.api.http.GitlabHTTPRequestor$1.fetch(GitlabHTTPRequestor.java:239)
at org.gitlab.api.http.GitlabHTTPRequestor$1.hasNext(GitlabHTTPRequestor.java:193)
at org.gitlab.api.http.GitlabHTTPRequestor.getAll(GitlabHTTPRequestor.java:161)
at org.gitlab.api.GitlabAPI.getProjects(GitlabAPI.java:560)
at org.jenkinsci.plugins.gitlab.GitlabRepository.getProjectForPath(GitlabRepository.java:101)
at org.jenkinsci.plugins.gitlab.GitlabRepository.checkState(GitlabRepository.java:42)
at org.jenkinsci.plugins.gitlab.GitlabRepository.init(GitlabRepository.java:33)
at org.jenkinsci.plugins.gitlab.GitlabMergeRequestBuilder.build(GitlabMergeRequestBuilder.java:60)
at org.jenkinsci.plugins.gitlab.GitlabBuildTrigger.start(GitlabBuildTrigger.java:64)
at org.jenkinsci.plugins.gitlab.GitlabBuildTrigger.start(GitlabBuildTrigger.java:23)
at hudson.model.AbstractProject.submit(AbstractProject.java:1804)
at hudson.model.Project.submit(Project.java:225)
at hudson.model.Job.doConfigSubmit(Job.java:1348)
at hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:772)
at java.lang.invoke.MethodHandle.invokeWithArguments(Unknown Source)
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:396)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:408)
at org.kohsuke.stapler.interceptor.RequirePOST$Processor.invoke(RequirePOST.java:77)
at org.kohsuke.stapler.PreInvokeInterceptedFunction.invoke(PreInvokeInterceptedFunction.java:26)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:212)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:145)
at org.kohsuke.stapler.MetaClass$11.doDispatch(MetaClass.java:535)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:747)
Caused: javax.servlet.ServletException
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:797)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:878)
at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:280)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:747)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:878)
at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:280)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:747)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:878)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:676)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:873)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1623)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:154)
at hudson.plugins.locale.LocaleFilter.doFilter(LocaleFilter.java:42)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at javax.servlet.FilterChain$doFilter.call(Unknown Source)
at com.ceilfors.jenkins.plugins.jiratrigger.ExceptionLoggingFilter.doFilter(ExceptionLoggingFilter.groovy:29)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at jenkins.telemetry.impl.UserLanguages$AcceptLanguageFilter.doFilter(UserLanguages.java:128)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:157)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:99)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:90)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:540)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:146)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:524)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:257)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1701)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:255)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1345)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:203)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:480)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1668)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:201)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1247)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:144)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.Server.handle(Server.java:502)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:370)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:267)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:305)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:103)
at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:117)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:333)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:310)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:168)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:126)
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:366)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:765)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:683)
at java.lang.Thread.run(Unknown Source)