Hi,
Thanks for your time and answer. We've reduced the situation until we found it. It was a memory leak in a 3rd party library (devexpress) that we use.
There was no scenario to reproduce, only loading our addin on startup was enough to cause the issue.
We only wanted to see if it's possible to obtain a hint of where the issue might originate since we were investigating this for quite some time. It wasn't affecting the functionality but it was annoying for the users to get crash reports when closing Revit.