If you use a SFTP key file and its path contains letters with accents (àâéè, etc.), probably Unicode characters, then Opus fails to connect.
Spaces in the path don’t seem to be a problem.
Discovered on Opus 12.11.1
Confirmed. Thanks for reporting this.
We probably won't have a fix in time for 12.12, but should have one shortly after if not.
(Update: Fix is now written.)