Solved. See reply to " NSUIActivityDocumentMonitor throws a strange exception".

After a LOT of investigation into this issue, I found the solution - never subclass NSApplication, and never override the "run" method. In our custom run method we were getting the next event and sending it out, then updating windows. That worked fine up until macOS 15.2 when we began seeing the error described above. I realized that another of our apps that did NOT subclass NSApplication was not exhibiting the issue which led me to change our override - I simply commented out our override of the "run" method and everything works fine now. We still subclass NSApplication because it is literally used a lot in our specialized AppleScript support, but it was our no-longer-doing-the-right-stuff run override that caused the error.