It's clear that the diag.txt file is often locked because Zorro writes all the time into it. But it should not be locked when nothing is written. I'll inquire what may be the reason, and wil then get back to you.

Update: The diag file indeed remains locked during the whole session for speed reasons. So you must really stop the session for reading it, then restart the session. I've put reading it with the stop trick on our todo list. It will be implemented in the next beta.