some poeple are experiencing 500 error after updating the groups plugin. Reload your site with a backup and when you update it, do not do the groups. See if that works. If so, wait until the groups thing has been resolved then consider updating.
FYI if it does work, you can uninstall groups and reinstall it hten it works (for me anyway)
You do lose the data but if it isn't used much then nothing much lost.