Login fails when groups used and access token is not a JWT

[Corfus]

When you use groups in the Plugin Configuration and the receiving access_token is not a JWT the login fails, see logs and screenshot.

2023-09-17T21:05:17.307320691Z I, [2023-09-17T21:05:17.307178 #1]  INFO -- : Completed 500 Internal Server Error in 554ms (ActiveRecord: 6.9ms | Allocations: 4660)
2023-09-17T21:05:17.308166091Z F, [2023-09-17T21:05:17.308094 #1] FATAL -- :   
2023-09-17T21:05:17.308183781Z NoMethodError (undefined method `unpack1' for nil:NilClass
2023-09-17T21:05:17.308189501Z 
2023-09-17T21:05:17.308194071Z     str.unpack1("m")
2023-09-17T21:05:17.308198681Z        ^^^^^^^^):
2023-09-17T21:05:17.308202961Z   
2023-09-17T21:05:17.308207061Z plugins/redmine_openid_connect/app/models/oic_session.rb:178:in `user'
2023-09-17T21:05:17.308211231Z plugins/redmine_openid_connect/app/models/oic_session.rb:163:in `admin?'
2023-09-17T21:05:17.308215311Z plugins/redmine_openid_connect/lib/redmine_openid_connect/account_controller_patch.rb:143:in `oic_local_login'

When you don't use the groups, everything works.

I think the reason is written here: oic_session.rb

Here will be tried to decode the access_token when it exists.
This only works when the access_token is a JWT like in Keycloak, but the format of the access_token is not defined in the RFC and can be everything also a random alphanumeric string and then it fails.

When I remove the special code for Keycloak, it works like expected

Normally I don't work with Ruby, so perhaps I didn't understand the code correct and the real reason is on another location, but for me it looks very logical.