After upgrading the warnings plugin to 4.0 half of our projects were not visible in the dashboard any more.
In the system log we found for every missing project the following exception trace:
Apr 24, 2012 10:01:51 AM jenkins.InitReactorRunner$1 onTaskFailed
SEVERE: Failed Loading job HIS-REGIO-1.1
java.util.NoSuchElementException: No parser found for group: Eclipse Java Compiler
at hudson.plugins.warnings.parser.ParserRegistry.getUrl(ParserRegistry.java:142)
at hudson.plugins.warnings.WarningsDescriptor.getProjectUrl(WarningsDescriptor.java:89)
at hudson.plugins.warnings.WarningsProjectAction.<init>(WarningsProjectAction.java:47)
at hudson.plugins.warnings.WarningsPublisher.getProjectActions(WarningsPublisher.java:235)
at hudson.model.Project.createTransientActions(Project.java:208)
at hudson.model.AbstractProject.updateTransientActions(AbstractProject.java:624)
at hudson.model.AbstractProject.onLoad(AbstractProject.java:279)
at hudson.model.Project.onLoad(Project.java:88)
at hudson.model.Items.load(Items.java:115)
at jenkins.model.Jenkins$15.run(Jenkins.java:2438)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$6.runTask(Jenkins.java:839)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)

Switching back to warnings plugin version 3.27 solves the problem for us.