This problem started for me with v13 and I had to go back to v11. I have now tried v15 and still get the same error, so I thought it worth reporting.
Right-click on a csv file > Sign and encrypt > I select ‘Encrypt for others’ and click Sign/Encrypt
It pops up a message “The file …csv.gpg already exists. Overwrite?”
(The encrypted file did NOT exist before I started this process)
I click ‘Overwrite’ and get error
“Kleopatra: Could not remove file “//server…csv.gpg” for overwriting.: Input/output error (218136625)”
Checking, I find that the encrypted file actually exists with what looks like a random text suffix. I remove that and decrypt the file and it is all OK.
While documenting the above I noticed that I was encrypting a file on the network. So I tried again with a local file and that work fine.
So it looks like a permission issue with a network file. But I have full permissions on the file and folder in question. And v11 works fine. So this is not really a permissions issue but some bug.