Oh boy, got it now :-)
Unfortunately, changed too many factors as same Time, so i can't say what caused this behaviour.
Changes are:
Replaced Fritzbox against Draytek Vigor 165 as Modem
Disabled IPv6 on Lan and Wan Interfaces
Disabled IPv6 Firewall Rule under "Firewall: Rules: LAN"
Set "Prefer to use IPv4 even if IPv6 is available" under "System: Settings: General"
IMHO it was the Firewall Rule.
Thanks for Support.
Unfortunately, changed too many factors as same Time, so i can't say what caused this behaviour.
Changes are:
Replaced Fritzbox against Draytek Vigor 165 as Modem
Disabled IPv6 on Lan and Wan Interfaces
Disabled IPv6 Firewall Rule under "Firewall: Rules: LAN"
Set "Prefer to use IPv4 even if IPv6 is available" under "System: Settings: General"
IMHO it was the Firewall Rule.
Thanks for Support.