Please note that at this stage we don't officially support custom ROMs with Viber. Running Viber on a custom ROM results in various side effects (sometimes total lack of functionality), but we can't afford to support all custom ROMs out there. We are continuously working to adapt Viber to more and more ROMS, but meanwhile - you might want to contact the ROM developer for further support - they may be able to resolve this issue.
* By the way, have you tried testing Viber on a Stock ROM on that same device?
Do you have Jelly Bean (Android 4.1) ?