I have found the reason - Leave Reload Bar Up (DEVELRELOAD) is True. If I try change it to False and apply settings I get error “Ajax Web ERROR: When saving key DEVELRELOAD: parsererror”.
It looks like a bug:
I used firebug to check server response and found that it tells me referrer is wrong in response but the message is not visible for normal user. So, what I did:
Set CHECKREFERER=FALSE in /etc/amportal.conf
Set value CHECKREFERER to 0 in freepbx_settings table
This resolved the issue. I think it worth to set CHECKREFERER to 0 by default.