At that time I only uploaded my pub key created with GnuPG to Keybase. (Private key stayed @home)
Suddenly my key stopped working in OnlyKey WebCrypt App
After I imported my pub key from keybase into gpg, I saw my key had expired. Aha, hence the error in WebCrypt.
Renewing the private key on Keybase was easy to find, but the pub key update was pretty hidden. If someone has the same problem, solution:
gpg --export <Key ID> | keybase pgp update
gpg --export <Fingerprint> | keybase pgp update
In case someone really has his private key on keybase and needs to update it:
gpg --export-secret-key <Key ID> | keybase pgp import --push-secret