It sounds like you’ve set up everything correctly from the Looker Studio side!
I would think that the most likely issue here would be the Trusted Domains / Allowlist in Workspace ( Admin Console > Account > Domains > Allowlisted domains), but I expect Workspace support would have already asked you to check that.
Thank you for your confirmation. As you mentioned, our support contact did advise us to check the sharing settings. However, as I have described below, there are no issues with them.
The problem has not been resolved yet, and we are still unable to determine the cause, which is why I’ve contacted you for assistance.