Seems to work now. Was a strange slash in our group-names in keycloak.
Here the changed parts
oidc.scope=openid,profile,email,address,xwiki_groups
oidc.endpoint.userinfo.method=GET
oidc.user.nameFormater=${oidc.user.preferredUsername._clean._lowerCase}
oidc.user.subjectFormater=${oidc.user.subject}
oidc.groups.claim=xwiki_groups
oidc.groups.mapping=41ca13be-83bc-4444-8bd1-454f215dd1a3=/41ca13be-83bc-4444-8bd1-454f215dd1a3
# oidc.groups.mapping=MyXWikiGroup2=my-oidc-group2
# oidc.groups.mapping=MyXWikiGroup2=my-oidc-group3
# oidc.groups.allowed=
# oidc.groups.forbidden=
oidc.userinfoclaims=xwiki_user_accessibility,xwiki_user_company,xwiki_user_displayHiddenDocuments,xwiki_user_editor,xwiki_user_usertype,xwiki_groups