Thank you for your message.
This is not a bug in FileMaker 12 per se. There are a number of things that can cause this. Consult the following knowledge base articles to see if any of them apply:
Typically one, or more of the items discussed in these articles will fix this issue.
A-ha, thanks so much!! :) The first link that you gave me solved the problem for me!! The "fmiwp" extended privilege was assigned to my user account in all of my files, even though I don't use fmiwp. So I disabled that Extended privilege in all of my files, and that seems to have fixed the problem!! :) Thank you!! And that explains why the problem wasn't happening in FileMaker 13 -- because there is no Instant Web Publishing in FileMaker 13!
You know, I had actually seen that Knowledge Base article earlier today, but I totally ignored it because it said that it was for FileMaker 9 only. Perhaps that article could be updated to list all versions of FileMaker, including FileMaker 12 and 13?
However, here's something equally unusual… I have absolutely ZERO FileMaker processes running on the computer right now. No instances of FileMaker Pro nor FileMaker Server are running, and Activity Monitor shows that no FileMaker processes are running. BUT… take a look at the attached screenshot. When I go into Network Utility and run a localhost port scan for port 5003, it shows that port 5003 is open on my machine. Is this normal? I'm not super-concerned about this because I'm no longer getting that error message within FileMaker Pro, but I just wanted to make sure that this is normal for this port to be open on my computer without FileMaker even running at all. Please see attached screenshot.
In any case, thank you so much for solving my problem!! :) Yay!!!!!!! :)