Hello @mrmarkuz and Nethserver forum,
I’ve provided Markus with a test S3 account from my S3 provider. Markus confirmed that Nethserver uses ‘subdomain style’ - https://{BUCKET}.{HOST}/${file_to_upload} connect to S3. My S3 provider uses ‘Path Style’ for S3 connections so this is the reason that Nethserver backup GUI cannot connect to my S3.
Nextcloud allows for both types of S3 connections so my hope is that Nethserver is able to update the backup GUI to also allow for both S3 connection types.
Many thanks to Markus for his continued support and diligence in confirming root cause.