I tried to reproduce this, just to make sure I didn't miss anything. Here's what I did:
Created a site under the root site. Allowed anonymous access on this site.
Created a group, did not give the group ANY permissions at all.
Added a membership request web part to this sub site. Edited the web part properties to use my new group.
Logged out and visited the page as an anonymous user. Created a user using the web part. No errors
Logged back in and checked the users in the group - the new user had been added to the group.
So it seems to be working properly. Are you sure that the version 1.2 was properly deployed? Maybe try a redeploy. Otherwise, i'm thinking there's something special with your configuration. Maybe, if you have it available and are handy with it,
deploy a debug version with Visual Studio and do a debug to root out the problem.