You used a trailing / on the tests above whereas you didn't on the first test. This may make a difference in that rclone doesn't need to check the root is a file if it has a trailing slash. Can you try it without the / and it should trigger the ERROR message I put it.
Either way though, it looks like stat does not work on the backend which is odd... Or maybe stat doesn't work on the root directory but it does elsewhere?
I think you've tried --sftp-use-fstat haven't you?
That was implemented following on from this forum thread
However I think this is likely a different issue though it is another IBM server so perhaps it is similar.
I removed the stat before doing a directory listing (can't remember why that is there anyway) which may at least get you a directory listing, so try this
v1.63.0-beta.7035.4da909a06.fix-sftp-error on branch fix-sftp-error (uploaded in 15-30 mins)