Well, I can narrow the root case. A Yubikey 5 was successfull installed and can be used. Then I started to test the OpenPGP card. I recognized, that by pressing F5 in Kleopatara a change between YubiKey and Smart Card happens. However, if I test it via command line, Yubikey does not change, although it is dismounted and the smart card is inserted. Probably therefore, the private key cannot be found. It should be mentioned that I have a computer with integrated smart card reader. First I configured the card, then the Yubikey. I started to test the Yubikey first. Therefore, I believe it is a mess in detection of smart card / Yubikey if used parallel.

Kleopatra recognizes the smart card, shows the correct version number and keys in the "smart card - management" window. In the Keylist I can´t find the key. Currently GnuPG 2.2.15 is installed. Do you know then version 2.3. will be released?

2.3 Release plan is around this summer. There will be a public beta sooner.

After re-start, the smart card will be recognized in proper way and it works. I assume it has something to do with using Yubikey and smart cards with different keys alternatively. The Yubikey was not found originally, so I modified the following:

log-file c:\..........\scdaemon.log
debug-all

gpgconf --kill scdaemon
gpg --card-status

and Yubikey was detected. I assume if a smart card is used after a Yubikey, the procedure has to be done manually again or the computer has to be re-started. As work around ok, but in future the program should recognize changes between Yubikey and smart card it automatically.

I'm interested if this works as you imagine with 2.3 I'm pretty sure werner worked on a problem like that.

So I've compiled the current 2.3 state. It's safe to use, I use it on Linux regularly but on windows it's very untested so please backup your %APPDATA%\gnupg directory anyway before you try it.

After you tried it out you can just reinstall Gpg4win to get back to the stable version again if you like.

Download is: https://heinecke.or.at/div/gnupg-w32-2.3.0-beta728_20190408.exe

Signed with my official GnuPG release key ( https://gnupg.org/signature_key.html ): https://heinecke.or.at/div/gnupg-w32-2.3.0-beta728_20190408.exe.sig

I´ll give it a try for sure! Probaly next weekend, so my feedback will be sent next week. Please, keep the file open. THANKS

By installation from version 2.3 an error occurred, I´ll send you a screenshot by e-mail. However, I have some comments to the current version which may also help: I have three keys, two on smart cards and one on a Yubikey. So long as only smart cards are used, it is no problem to change between the cards and they work fine. Problems occur, if a Yubikey comes in. (i) Not always a Yubikey is recognized by pressing F5. (ii) It the Yubikey is recognized and next a key from a smart card is needed, a computer restart is required.
I tried also command: gpgconf --kill gpg-agent
It was possible to change from smart card to Yubikey with the command. However, if the Yubikey 5 NFC was recognized, the only way to change back to the smart card was a restart of the computer.

So long I change between smart cards, I can do it multiple times. If a Yubikey is recognized and a smart card follows next it will not work. Most recently I face also problems to detect the Yubikey (Message: no such device), but Smart Cards still working fine.

gpg-connect-agent /bye followed by gpgconf --kill gpg-agent did not solve the situation.

Well, I deinstalled gpg 3.1.7 and reinstalled it. For some reason my two gnupg smart cards work fine, but my two Yubikeys cannot be detected anymore (no such device). But in the last weeks, they were deteced, only the switching between Yubikey and Smart Card made some trouble. That they cannot be recognized is new and makes real trouble. If you think it would maybe helpful, I can submit a scdaemon.log file by e-mail.