Following the suggestion from
https://docs.openstack.org/swift/latest/overview_auth.html I set
'log_level=debug' for authtoken.

Now I'm seeing more errors in the glance-api logs:

Apr 06 15:07:40 infra1-glance-container-99614ac2 glance-wsgi-api[1837]:
2021-04-06 15:07:38.197 1837 WARNING keystonemiddleware.auth_token
[req-ad8d0db9-b630-4237-9fff-d7ff282155d2 956806468e9f43dbaad1807a5208de52
ebe0fe5f3893495e82598c07716f5d45 - default default] Identity response: {"error":
{"code": 500, "title": "Internal Server Error", "message": "An unexpected error
prevented the server from fulfilling your request."}}:
keystoneauth1.exceptions.http.InternalServerError: An unexpected error prevented
the server from fulfilling your request. (HTTP 500) (Request-ID:
req-55767c13-a450-4f81-90ab-5a7af6b3f672)
Apr 06 15:07:46 infra1-glance-container-99614ac2 uwsgi[1822]: DAMN ! worker 13
(pid: 1836) died, killed by signal 9 :( trying respawn ...

Here's the full log: http://paste.openstack.org/show/804208/

The options you suggested @Dmitriy are still there in user_variables.yml:

glance_glance_api_conf_overrides:
  keystone_authtoken:
    service_token_roles_required: True
    service_token_roles: service

Kind regards,
Oliver