Fritz uses a "pre-loaded route set" to address the proxy. When using a domain in the Route header
Route: sip:sip.advancemedia.it;lr
openser fails to detect that this request is targeted to itself. (loose_route() compares the URI with "myself" from core) Thus, you have to add the domain as alias:
alias=sip.advancemedia.it
this should solve your problem
regards klaus