500 Internal Server Error

Just testing out the latest dev release, but i'm having a strange issue which involves other modules and a 500 internal server error.

I set up auto assign role module with a custom path (eg. signup/tips). when a user visits that page the correct form is displayed, but when clicking submit a "500 Internal Server Error" occurs. I tested without mimemail - and it works without a problem. i tested with the older dev version of mimemail - and it works (though other problems consisting of blank / n/a email are there).

This might also have something to do with the Rules module, which triggers on signups. from what i can tell - something goes wrong even before the auto assign role is assiging the correct role.

I know this is a very specific use case - but it's annoying none the less...

Any ideas on what can be checked?

Thanks!