sometimes no slashes are necessary, for example on a single-level subfolder you can just type folder.
If you want to see what an S3 URL should look like:
s3://bucketname/prefix
and you can compare that to Target URL on the Commandline screen. I “think” the only double slash is supposed to be the one at s3://, anything in the middle is single forward slash, and I’m not sure of end.
You can also take that URL (Export As Command-line can also be used if you want it quoted for you) to:
Duplicati.CommandLine.BackendTool.exe to see if you do operations without losing the first character…