No, there are no errors in the log file - I did a network sniff and after the timeout I only receive one strange ? character in the content field of requests that timed out.
At this moment in time I don't think it's anything server related. I did a couple of other tests and found out that if I request content via file_get_contents() from inside the internal network (/27 mask) I do get a response. Only if a request is routed via the gateway which appears to be running a firewall am I running into the timeout problem.
$ netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
XX.XX.122.32 0.0.0.0 255.255.255.224 U 0 0 0 eth4
0.0.0.0 XX.XX.122.36 0.0.0.0 UG 0 0 0 eth4
This is strange since applications like lynx or wget are working without problems for internal and external URLs and they should be working on the same ports. The only conclusion I can draw from this is that the firewall is somehow capable to filters content on the application level and recognized a request made by apache and somehow blocks it. I wasn't aware that firewalls work on this level.
In any case I have contacted my network provider and asked him to check it out and will let you know what comes out.