The workaround for me was to manually copy the eclipse plugins jar files that came with the UIMA distribution to the plugins directory of my Eclipse installation as described in sections 3.1.5 and 3.1.6.1 of the "Overview and Setup" documentation.
For the benefit of anyone else who hits the same plugins manager error, the best description I found is is a thread<http://www.eclipse.org/forums/index.php/t/7409/>in the Eclipse forums which basically says that it is a low-level Eclipse error that end users should not see.
