Hi @PTL_SQL
1.From what I've searched, "Error: 18456, Severity: 14, State: 38. Login failed for user '<x>'." is generally caused by the failure to open the database specified in the login properties or the database '<database>' requested by the login name. For more information check out this link: https://sqlblog.org/2020/07/28/troubleshooting-error-18456.
2.gMSA is not officially supported as a report server service account. They should only be used at your own risk.
This is stated in the documentation on SSRS: https://learn.microsoft.com/en-us/sql/reporting-services/install-windows/configure-the-report-server-service-account-ssrs-configuration-manager?view=sql-server-ver16#choosing-an-account
Best regards,
Aniya