"I've managed to reproduce the "Failed to authorize" issue once: "Default encryption" is enabled for an iDrive bucket.
It seems that they've implemented encryption in a way which modifies the ETag / MD5 hashes on the S3 protocol side. S3Drive performs strict ETag validation which is required for not yet released syncing features. After we write an object to iDrive E2, we expect ETag to match content of file that we've sent to the server (file integrity), instead they seem to return ETag of their encrypted blob.
We will think of workaround, e.g. by disabling syncing features (and notifying user) if we detect ETag issue instead of preventing user from logging in by displaying: "Failed to authorize", however before it's there the only way is to disable iDrive encryption.