Windows 7/10. Kleopatra 3.1.15.

When trying to encrypt file directly in UNC file share, then it fails with input/output error 218136625. Decryption works fine in UNC file share. Kleopatra UI shows all UNC path backslashes as forwardslashes which might be a cause of the issue.

User permissions on file share are correct (full control).

Put a file in UNC share, eg., \\server\share\file.pdf
Right click on file and choose "Sign and encrypt"
Kleopatra shows output as //server/share/file.pdf.gpg
Press "Sign / Encrypt"

Kleopatra asks to overwrite already existing file at //server/share/file.pdf.gpg
Press "Overwrite" or "Overwrite All"

Kleopatra shows "OpenPGP: All operation completed" and "Sign/encrypt failed"
In UNC share there is a file named file.pdf.gpg.RANDOM