Thanks for posting!
Since we know it's a user issue, let's try moving all fonts from the user font folder, instead of just Arial. Open /Users/<your username>/Library/Fonts/ in finder and move all of these fonts to a folder on your desktop. Then, launch FileMaker again and let us know if it still gives you this issue.
Removed all fonts to a folder on desktop, from Users/<username>/Library/Fonts
No change. Filemaker wasn't getting fonts from there.
Then I removed all fonts to a folder on the desktop, from System/Library/Fonts
This changed things....but now Filemaker sees a very limited number of fonts...including Arial.
Finder search for some of those fonts finds only font suitcases with those names in a Microsoft Office folder.
Finder search for "Arial" returns only fonts that are in "disabled" folders on the desktop.
So, Filemaker is finding fonts somewhere else!
I had also found (and trashed) a very old file from years ago, "FM Pro Fonts". I recall v.2 or 3 used to get fonts from this file.
So, still not solved!