Hi.
The Amazon S3 buckets are case sensitive.
You can create 2 buckets named “MyBucket” and “mybucket” and they will be considered as 2 separate ones.
Apparently, the developer of this plugin was oblivious to this fact and try to connect using all lower case.
Sloppy.
The solution is to create a bucket with all lower case in the name, select it in the plugin and it will work.
(And when done, send an email to the developer of this plugin and tell them to to fix their mistake).