After your helpful hint I found the cause of the Access Violation - or at least, I found what I suspect is the cause, and it is now gone
As part of my testing for the earlier issue I had installed to a new path, and as a consequence the "User Data Path" in the Editor/Paths panel of the Preferences was pointing to a non-existent directory. As soon as I pointed that back to somewhere that exists, that problem disappeared.
Now, I am not sure what the User Data Path is for, and it certainly was not pointing to either the directory with my programs or the directory containing the NXT.DAT file, but once I changed the User Data Path, the Find Brick command stopped working again. I resolved that by copying my NXT.DAT file to the directory I had specified for User Data Path.
I hope that is adequate information for you to track down the issues you mentioned you have been seeing as well, and to implement some error trapping or workarounds. Let me know if there is anything elsse I need to add to the above information.
(For the record, I was using the MDI mode, but I frankly doubt that was relevant.)