Re: 404 for groups
I just did a search on all of mu and bp and nothing contains ‘DTD HTML 4.0 Transitional’. I don’t think that’s coming from mu or bp. What plugins do you have running in /mu-plugins and /plugins? Disable all and try again.
That is a suspiciously blank html file. Normally an error gets you nothing or part of the page.
You can access all other group type functions. ‘my account’ groups etc..? Re-upload the dir /mu-plugins/bp-groups
(thrashing around aimlessly here)