I just ran into this bug while testing for Grails 1.3.8. Nieraj helped debug it.
We discovered that it is a bug which rarely occurs. It is triggered by duplicate entries in the downloaded list of plugins.
Once you hit the problem, it will keep re-occurring for you because you will have a cached copy of the plugin list file on your disk. So even if the list changes later on on the server and other people may not be seeing the problem, the unlucky guy who has a cached copy with duplicates... will have a problem.
Refreshing the list of plugins may make the problem go away (assuming the list has been fixed on the server).
since you can't actually open the plugin manager. You may need to manually delete the plugin list files from your .grails folder to clear out the file.
I am now adding some code to fix the problem (i.e. so STS handles the duplicate entries better).