I had the same issue. From an internet search found there is a bug where the buttons on a form loose link to their vba code. At least that is how I understood it. In any event its intermittent. Here is the way to "fix" it until it happens again the next time ...
Opened the addcb file, compiled the vba code, then recreated the addce file. It might fix it the first time or the second or ... you get the idea; test it, repeat until it resolves itself. Annoying ... but what you gonna do?
Good luck.