This sounds very similar to an issue with FileMaker Pro v15 when opening files under certain conditions. Have at these links and see if you're affected by the same bug:
Nothing in your post relates to FMS crashing. Just saying.
Sorry, I've been busy using the program in question. Although my original post said FMS crashes, it does not. I know this because other computers continue to use the server. The message said it crashed. I only have one user using FMP15 at the present time and she was the only user having problems. She is on vacation so I've been using FMPA15 and FMPA14 with the programs on the server to do her job. FMPA15 gives me problems by crashing when I am moving between files two or three times a day just as she had. When using a pull down menu it may not alway work. I get the spinning ball for a minute and then it crashes or gives me nothing. FMPA14 gives me the value but takes longer than it used to. When opening the program on the server, FMPA14 starts out slowly with a broken up looking picture and then clears up. It has been like that for the last year but it has run properly. Right now I am not feeling confident about using Filemaker Pro. I've trashed the FMP15 from my order entry desk and loaded FMP14 on it. When the operator comes back next week I'll see what happens. The same set of files ran very smoothly for years. FMP11 probably ran the best. I never had any problems other than with FMP7.