When I select "Nachricht als neu bearbeiten" ("Edit mail as if new ?"), I receive a warning message (attached) that doesn't seem to relate to what I just did. Also it has a lot of smaller writing errors (marked). The last half sentence must not stand alone.
Regardless, the mail window opens. On pushing "Send" the mail won't be sent, window stays open, a circle of dots notifies waiting time. This somewhat locks the whole cryptography. I can't reliably save this as a draft and using such a draft will repeat the problem.
If the warning message is correct (by a little modification) and opening of the unsendable mail window can't be stopped, the warning should be stronger! Any text written in the mail may be lost after restarting.