@bjoernneumann
It happened when coming from 1.9.0 to 2.0.31 and also when coming from 1.9.0 to latest (2.1.7).
The Setting concerning downloadbehavior was “open after download complete” --> altered to “Ask for every download”.
After changing the KD still opens the attachment when download was complete, although it was set to “Ask for every download”.
Will try “F12” on the next client if the problem occours again.