Hi Daniel,
Please check if you have enabled aim authentication for the app's user in the database. Please find an example in line 35:
Also, you can upgrade to the last Operator version since it fixed a PEM authentication-related issue:
https://docs.mendix.com/releasenotes/developer-portal/mendix-for-private-cloud/#2.20.1
If nothing works, you can spin the reference deployment, the above Github project, to check a working IRSA configuration.
Good luck!
Carlos.