Hi akos, welcome to the forum
Good post!
I use SFTP daily and haven’t encountered that error, so my best guess that it has something to do with your SFTP server (in combination with rclone).
Somebody may be inclined to take a deeper look if you can reproduce the issue on a well-known publicly available SFTP server with many users. Preferable using a simple example reading from the local file system.
Otherwise, my best advice is to continue using --sftp-disable-concurrent-writes to get maximum stability. And then try to remove --sftp-disable-concurrent-reads to avoid the ERROR message triggered by #5197
Probably not the answer you were hoping to get, but that is the best I can offer.