Too much delay in IAX2 calltoken timestamp from address

Hi all,
did anyone of you ever got with this message
handle_call_token: Too much delay in IAX2 calltoken timestamp from address X.X.X.X
???

this is my scenario:
asterisk (public ip) <—internet----> FW <-lan-> asterisk
i managed to get an iax2 trunk between my asterisks but after few calls iax registration went to hell [ :smiling_imp: ] giving me the error in subject.
googoling doesn’t help me…

ideas, suggestions, comments?
thanks,
Francesco.