Hi again, @bernhard
I have been following the discussion at dev.gnupg.org regarding this problem ( ⚓ T7339 Kleopatra: Cannot decrypt packets with hybrid cipher without using symmetric passphrase ).
I noticed that one of the developers wrote : “I pushed my patch for gpg, since it does not break anything, just allow empty passphrase input (to skip).”.
But that is a bit funny / strange. Because when I use the line command ‘gpg –decrypt …’ it already works this way - i.e. I’m first prompted for the symmetric key, and if I either push the cancel button, just push ‘Ok’ without entering a passphrase or if I enter a wrong passphrase and push ‘Ok’ I always get prompted for the gpg key passphrase afterwards.
The problem with the ‘missing second prompt’ only occurs when I try to decrypt a file encrypted this way via Kleopatra. So without knowing anything about Kleopatra internals I would have expected that a fix for Kleopatra was necessary, not for ‘gpg’ itself.
Unfortunately signing up for ‘dev group’ has been disabled for ‘newcomers’ - so in case you agree with ‘my confusion’ could you perhaps write a note regarding this in the thread (I can see that you are monitoring this thread, so you must have a user there 
)
Best regards
Frank