In my Fritzbox it states the following:

IPv6-Adresse: 2a02:8070:600::14b6:c7******, Gültigkeit: 100465/86065s
IPv6-Präfix:  2a02:8070:62c:3200::/56, Gültigkeit: 100464/86064s

I am not able to connect to IPv6 Addresses from computers configured by the fritzbox, because they get an address with prefix 2a02:8070:62c:3200::/56 but somehow the fritzbox does not route those addresses.

Is this because the IPv6-Address is not in the prefix range?

The address of the Fritzbox responds to pings, but my computers reached via various addresses from the prefix range do not (:

PING 2a02:8070:62c:3200:28d****(2a02:8070:62c:3200:28d****) 32 data bytes
From 2a02:8070:600:0:14b6*** icmp_seq=0 Destination unreachable: Administratively prohibited
From 2a02:8070:600:0:14b6*** icmp_seq=1 Destination unreachable: Administratively prohibited
From 2a02:8070:600:0:14b6*** icmp_seq=2 Destination unreachable: Administratively prohibited
From 2a02:8070:600:0:14b6*** icmp_seq=3 Destination unreachable: Administratively prohibited

blocked ping this is "view" from outside

From inside the ping6 looks like this:

traceroute -6 heise.de
traceroute to heise.de (2a02:2e0:3fe:100::8), 30 hops max, 80 byte packets
1  fritz.box (2a02:8070:62c:3200:****)  0.787 ms  1.424 ms  1.702 ms
2  * * *
......
30  * * *