I've just upgraded to STS 3.6.1 and started encountering this error:

'Building workspace' has encountered a problem
Errors occurred during the build.

The detailed message says:
Errors occurred during the build.
Errors running builder 'Spring Project Builder' on project 'XXX'.
org/springframework/security/config/InternalInterceptMethodsBeanDefinitionDecorator

A clean build seems to work around this. I found this issue:

http://forum.springsource.org/showth...itionDecorator

which discusses cleaning up the plugins folder as a fix here:

https://jira.springsource.org/browse/IDE-1099

Created a temp directory and moved the "duplicate" directories (3) and files (24) below which all were from 3/30/2011 and had a more recent version from 5/6/2011.

Still seeing same problem - everything else seems OK, clean, build is a work-around. Any ideas to make it go away?

dir:
com.springsource.sts.ide.ui_2.6.0.201103161000-RELEASE
com.springsource.sts.server.tc.reloading_2.6.0.201 103161000-RELEASE
com.springsource.sts_2.6.0.201103161000-RELEASE

files:
com.springsource.sts.bestpractices_2.6.0.201103161 000-RELEASE.jar
com.springsource.sts.config.core_2.6.0.20110316100 0-RELEASE.jar
com.springsource.sts.config.flow_2.6.0.20110316100 0-RELEASE.jar
com.springsource.sts.config.ui_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.content.core_2.6.0.2011031610 00-RELEASE.jar
com.springsource.sts.core_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.frameworks.core_2.6.0.2011031 61000-RELEASE.jar
com.springsource.sts.frameworks.ui_2.6.0.201103161 000-RELEASE.jar
com.springsource.sts.ide.configurator_2.6.0.201103 161000-RELEASE.jar
com.springsource.sts.ide.help_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.ide.metadata_2.6.0.2011031610 00-RELEASE.jar
com.springsource.sts.ide.osgi_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.maven_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.quickfix_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.roo.core_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.roo.ui_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.server.commons.vmware.core_2. 6.0.201103161000-RELEASE.jar
com.springsource.sts.server.commons.vmware.ui_2.6. 0.201103161000-RELEASE.jar
com.springsource.sts.server.insight.ui_2.6.0.20110 3161000-RELEASE.jar
com.springsource.sts.server.tc.core_2.6.0.20110316 1000-RELEASE.jar
com.springsource.sts.server.tc.ui_2.6.0.2011031610 00-RELEASE.jar
com.springsource.sts.server.tc.vmware.core_2.6.0.2 01103161000-RELEASE.jar
com.springsource.sts.server.tc.vmware.ui_2.6.0.201 103161000-RELEASE.jar
com.springsource.sts.ui_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.vmware.core_2.6.0.20110316100 0-RELEASE.jar
com.springsource.sts.vmware.ui_2.6.0.201103161000-RELEASE.jar
com.springsource.sts.wizard_2.6.0.201103161000-RELEASE.jar