Same thing happened to me. There was a message "your fare is no longer available".
My neighbour who is a ticket agent, wasn't able to issue these tickets as well.
I was able to book them through booking.com - they use "gotogate" to issue the tickets.
BUT - when Oman cancelled one of the segments - it was A PAIN to try to rebook these tickets....
Afterall all went well through Omani call centre in Muscat (a supervisor solved it all with one click)