Actually it's more a solution than a question.
I made a change to my Sitecore config and suddenly I could not login anymore with the not so helpful error message:
[InvalidOperationException] contextDatabase Origin: Sitecore.Forms.Core.Dependencies.DefaultImplItemRepository..ctor
I searched for similar problems, but only found CD-related issues with WFFM-configuration. As the error occurred on my development server that couldn't have been the problem.
Has anyone encountered similar issues?
Own Answer:
After some reflective digging through the Sitecore code I found the problem:
Origin was a faulty entry in an event handler where I copied the wrong Namespace reference.
<event name="publish:itemProcessed" help="Receives an argument of type ItemProcessingEventArgs (namespace: Sitecore.Publishing.Pipelines.PublishItem)">
Obviously the error bubbled up to the above mentioned class where it was finally caught.
So much for conclusive error handling.
Cheers