Hi Krikor, many thanks for your reply.
I've verified the correct setup several times and in various order. Already ran the akeeba admin tools but it still doesn't work…

I also used different color-shemas for joomla component and K2 component to identify which one is loading - but it always shows the color according to the setting in K2.

Is is possible that a wrong setting/configuration of the server causes the issue? I am currently creating a backup of the site and will host in on another server...

In fact it seems to be problem on the server config. A backup, created with Akeeba Backup and restored on another server, works properly.

..any idea how to get into troubleshooting?

Thanks
Martin

I would check the path to my /log and /tmp folders first.
Then the PHP version and ask if my host is blocking any commands that Joomla! needs in order to function properly.

Finally I would also check the owner and the group that my folders belong to, in order to make sure that there aren't any conflicts.

Ok, I verified the path to /logs and /tmp are correct. PHP version is 5.3 and I am using iMSCP as hosting software. One month ago, it worked properly.
I restored the backup, which is working on that other server, to a temporarily created "domain" (using imscp) and it didn't work. That shows me that the problem is related to the server configuration.

What's the fastest way to recover? I guess to save backups of the sites, re-install the server, and import the sites again? :dry:

I would use Akeeba Backup to get the entire site and DB into a .zip file.
I would then test them locally and then redeploy the server.

Finally I solved the problem. It was something completely different:

For some reason, my server resolved the domain google.com in an ipv6 adress instead of v4.
ping was OK, but curl -v google.com resolved as ipv6 adress first:

root@abc:# curl -v google.com
* About to connect() to google.com port 80 (#0)
* Trying 2a00:1450:4001:c02::8b... Timeout
* Trying 173.194.70.101... connected
* Connected to google.com (173.194.70.101) port 80 (#0)

Therefore, the fsocketopen() method did neither get any response nor a timeout.

For a temporary solution, in the file administrator/components/com_k2/lib/recaptchalib.php I replaced the url of RECAPTCHA_API_SERVER and RECAPTCHA_VERIFY_SERVER with one of google's ip's:

Address: 173.194.70.138
Address: 173.194.70.113
Address: 173.194.70.102
Address: 173.194.70.100
Address: 173.194.70.101
Address: 173.194.70.139