I am seeing this too, trying to add a 'secret text' credential,

credentials 1.24
credentials-binding 1.6

jenkins 1.609.3

+1 Get this too.

I was still getting this error during the week.

Upgraded Credentials Binding Plugin 1.4 to 1.7 and the error went away.

Jenkins 1.625.3
Credentials Plugin 1.26
Credentials Binding 1.7

I'm going to mark this Resolved.

Fix: Installed Credentials Binding 1.7

This same thing is happening to me with:
Credentials Binding Plugin 1.8
Credentials Plugin 2.1.4
Jenkins 1.625.3

This happens whenever I choose a credential to use as an env variable (both conjoined and separated). The issue is resolved by downgrading Credentials Plugin to 1.18

javax.servlet.ServletException: java.lang.RuntimeException: Failed to instantiate class org.jenkinsci.plugins.credentialsbinding.impl.SecretBuildWrapper from {"bindings":{"usernameVariable":"machine_user","passwordVariable":"machine_pw","credentialsId":["60005674-a10b-413e-b2b5-e217dd960c21",""],"stapler-class":"org.jenkinsci.plugins.credentialsbinding.impl.UsernamePasswordMultiBinding","$class":"org.jenkinsci.plugins.credentialsbinding.impl.UsernamePasswordMultiBinding"}}
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:796)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.MetaClass$6.doDispatch(MetaClass.java:249)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:686)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1494)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:123)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1474)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:499)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:533)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:428)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
at org.eclipse.jetty.server.Server.handle(Server.java:370)
at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:489)
at org.eclipse.jetty.server.AbstractHttpConnection.content(AbstractHttpConnection.java:960)
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.content(AbstractHttpConnection.java:1021)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:865)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:668)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.RuntimeException: Failed to instantiate class org.jenkinsci.plugins.credentialsbinding.impl.SecretBuildWrapper from {"bindings":{"usernameVariable":"machine_user","passwordVariable":"machine_pw","credentialsId":["60005674-a10b-413e-b2b5-e217dd960c21",""],"stapler-class":"org.jenkinsci.plugins.credentialsbinding.impl.UsernamePasswordMultiBinding","$class":"org.jenkinsci.plugins.credentialsbinding.impl.UsernamePasswordMultiBinding"}}
at hudson.model.Descriptor.newInstance(Descriptor.java:579)
at hudson.util.DescribableList.rebuild(DescribableList.java:177)
at hudson.model.Project.submit(Project.java:234)
at hudson.model.Job.doConfigSubmit(Job.java:1206)
at hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:796)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:298)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:161)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:96)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:121)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
... 48 more
Caused by: java.lang.NullPointerException
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:645)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:404)
at org.kohsuke.stapler.RequestImpl.instantiate(RequestImpl.java:697)
at org.kohsuke.stapler.RequestImpl.access$200(RequestImpl.java:81)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:600)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:638)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:404)
at org.kohsuke.stapler.RequestImpl.instantiate(RequestImpl.java:697)
at org.kohsuke.stapler.RequestImpl.access$200(RequestImpl.java:81)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:600)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:404)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:400)
at hudson.model.Descriptor.newInstance(Descriptor.java:570)
... 62 more