Finally found the solution just 2 minutes after posting - Clearly a FM14 Bug.
In case you have the same problem:
1. Disable automatic resize of the button bar (by using the inspector),
2. switch back to browse mode,
3. switch back to layout mode and enable the automatic resize
4. be happy.
Possible bugs should be posted to Report an Issue (see tabs at top of screen) where they can be seen, confirmed and logged by FileMaker Inc. personnel.
Thanks for this advise, I'll report it.