S2S Jabber problems

For some reason the DH Jabber servers don’t talk to njs.netlab.cz - anyone from that domain on my contact list appears offline. Can someone give me a hint? Is this a routing or DNS issue? Or maybe a Jabber dialback authentication problem. Either way, pretty annoying and has been ongoing for a while now, so any help appreciated. Connecting to a different server such as jabber.org seems to have no issues with netlab.