it seems, that the old kibana\_* like e.q. kibana_7 has type:doc while the new kibana_8 has type:_doc. When now using an index pattern of kibana\_* you run into the strict rule.
On my testsystem I had .kibana_8 and .kibana_7 where .kibana_8 seems to be a migrated version of kibana_7. I bypassed this issue by executing

    curl -XGET localhost:9200/.kibana_7/_search >kibana_7
    curl -XDELETE localhost:9200/.kibana_7

After Kibana restart I have access, but lost the Kibana Index patterns.  I will wait for a fix before upgrading my prod system.