@ikloecker
Thank you.
So it's a different issue.
Sorry, I was confused because after solving the gpg: can't connect to the agent I instantly got gpg: problem with the agent: End of file.

gpg: can't connect to the agent: IPC connect call failed

This problem with portable mode in Windows can be solved by creating additional gnupg folder near bin, home, share.
I don't know why, but gpg-agent v2.3.2/2.2.30 in Windows in portable mode creates files S.gpg-agent.* in gnupg, not in home folder. And it doesn't work without gnupg folder.

Few more logs from 2.3.2 and 2.2.29 (for comparison):

I'm not sure that the portable mode is a culprit here.
Something is very wrong with gpg-agent/pinentry.
Even symmetric decryption doesn't work in 2.3.2/2.2.30:

The same problem. Portable mode is completely broken in v2.3.2 and v2.2.30 on Windows.

The bugreport was about "use existing key" selecting keygrips and I did try to use "change-usage" (for NIST P-256).