I have defence+ set up to block all executables and DLLs in all writeable directories. This is to mimic a software restriction policy but be more flexible. I have a thundirbird add-on that uses a DLL from the user directory and it was not working as I assume the DLL was blocked. Installing the add-on globally (under program files) made it work.
Should this blocking be logging? It would have been a lot easier to find the problem if it had been logged.