I feverishly installed the latest Springsource Tool Suite release today – 2.5 RC1. Unfortunately it fairly swiftly started giving me the error shown below as I worked on my Grails project. It was popping up really frequently and would be a show-stopper if I couldn't fix it.
It became apparent that it does this every time I saved a Groovy file that required my running Grails app to reload. It seems that STS didn't like the files in the 'target' folder changing as it recompiled, especially since I have auto-refresh turned on in STS. I worked around this by setting up a rule for my project to exclude the target folder.
Right-click on the project, selected "Properties…" then Resource > Resource Filters. Add a filter to exclude the folder "target".