I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue:
Enterprise App Apple Business Manager Renew SCIM-Token Connection Test Error "SystemForCrossDomainIdentityManagementCredentialValidationFailure"
Solution:
Confirmed by @Florian Bartsch
It seems the issue was resolved after some time by refreshing the page.
If I missed anything please let me know and I'd be happy to add it to my answer, or feel free to comment below with any additional information.