Thanks to all for your help. mipiano did the job for me:
System Preferences -> Keyboard
...and added other, new shortcuts for FileMaker Pro
Decrease font size: cmd-shift-minus
Increase font size: cmd-shift-plus
so that the default cmd-< and cmd-> for prev/next window do work again.
As an unexpected and unwanted side effect new menu items appeared on their own, adding shortcuts for increase/decrease by 1: option-cmd-> / <
But I don't mind about those...
