Upgrading server from 5.39 to 6.0.2 fails

Troubleshooting
5

More than two months later, I tried to upgrade from 5.39 to the fresh version 6.30. Hoped, that something has changed in the meantime, but it still fails:

{
   "timestamp":"2022-01-20 14:47:37.689 +01:00",
   "level":"fatal",
   "msg":"Failed to alter column type. It is likely you have invalid JSON values in the column. Please fix the values manually and run the migration again.",
   "caller":"sqlstore/store.go:932",
   "error":"pq: default for column \"notifyprops\" cannot be cast automatically to type jsonb",
   "tableName":"ChannelMembers",
   "columnName":"NotifyProps"
}

I would really appreciate any help here, I have no idea how this can be solved.

Thanks in advance
Markus