So I started to deactivate some functions. First I thought it is an IP issue but at least I found the cause: I had a setting for 'Away Schedule' (20:00 to 07:00) - the server time and location (same in Joomla) is right for 'Europe, Berlin'. Everything is correct but Admin Tools recognized the UTC time only. Example: I tried to login at 7:30 local time but AT has as time: 05:30. Then of course the access was prevent.
But why? Everything was okay before the Joomla upgrade.