It was a bunch or work and we are still not able to pass Unicode strings on the command line. Will eventually be done. At least people in Asia can now use their regular Windows account with gpg.

Yes sure. --debug ipc should give you some insight why gpg does not thing the key is on the card.

Ingo, can you please check? I guess we are not affected because Kleo already checks for an empty string. But dkg's suggestion sounds good to me.

Thanks Werner! Seems like an important step!

After patching the above mentioned if-clause the command fails on the first try, but it succeeds on the second try

$ gpgconf --kill all

A fix has been released; see T5052.

I change this to a feature request: Allow several processes to run public key decryption using the same set of private keys.

Note that you actually run 30 independent processes with gpg 1.4 but with gpg-agent there is just one process to handle the private key operations (decrypt). To utilize more cores you need to setup several GNUPGHOME with the same private keys.

I think that it is not gpg-agent but pinentry which causes millions of futex syscall errors.
For interactive use case, pinentry may be the point of contention.
I might be wrong if your key is not protected by passphrase.

Indeed, I think this might be a driver problem.