It seems internally generated requests via function "tmb.t_request_within()" do not contain Max-Forwards header and there therefor not RFC3261 compliant. This can be manifested by many internally generated requests, an easy one to test is with dispatcher ping.
Steps to reproduce:
1) Configure dispatcher module to use OPTIONS probes, with OpenSIPS#2 box in dispatcher list
2) On OpenSIPS#2, call "sipmsg_validate("sh")" early in route block before handeling OPTIONS
3) Watch as the sipmsg_validate fails, debug output attached
Patch tmb.t_request_within to include maxforwards header.