Thank you for your advice, I got a little bit forward.
I expended my topology with another pc - another vpn client - and I got these two vpn clients working, but somehow I cannot get the server to work properly. The server remains always in state Init/Other.
I can see with tcpdump, that every pc is sending the hello-message, but the server is missing the neighbor list:
08:48:55.791063 IP (tos 0xc0, ttl 1, id 15221, offset 0, flags [none], proto OSPF (89), length 64)
Router-ID 10.29.0.1, Backbone Area, Authentication Type: none (0)
Options [External]
Hello Timer 10s, Dead Timer 40s, Mask 255.255.252.0, Priority 1
Designated Router 10.29.0.1
08:49:02.449351 IP (tos 0xc0, ttl 1, id 6717, offset 0, flags [none], proto OSPF (89), length 72)
Router-ID 192.168.21.1, Backbone Area, Authentication Type: none (0)
Options [External]
Hello Timer 10s, Dead Timer 40s, Mask 255.255.252.0, Priority 1
Designated Router 10.29.0.4, Backup Designated Router 10.29.0.8
Neighbor List:
192.168.21.17
10.29.0.1
08:49:02.854749 IP (tos 0xc0, ttl 1, id 9690, offset 0, flags [none], proto OSPF (89), length 72)
Router-ID 192.168.21.17, Backbone Area, Authentication Type: none (0)
Options [External]
Hello Timer 10s, Dead Timer 40s, Mask 255.255.252.0, Priority 1
Designated Router 10.29.0.4, Backup Designated Router 10.29.0.8
Neighbor List:
192.168.21.1
10.29.0.1