Ok, I got a very easy problem---some of my objects are destroyed in the global cleanup phase, not when I undef their parent objects.
I already checked all global variables, they're not guilty. I also checked for module scoped 'my' variables, no luck. I checked the parameters and local variables of the run() method, they're clean, too.
But still there are (at least) two objects that are destroyed to late. And, they're not just being destroyed to late, their DESTROY() also fails, leaving orphan locks in the database.
So, my main question is: Is there any module that can actively search for references for a given classname? Or just search for all CVs that still exist and give their content and owner?