I think I have a problem with topoh, but I can't identify it.

Kamailio 5.6.6, used as a stateless proxy, receives a 487 from a phone and propagates it correctly.

Next comes the ack that should be forwarded back to the phone, but instead it doesn't forward it and produces the following error "pv_get_ruri(): failed to parse the R-URI"

The error is generated by xlog("...$ru from $fu...\n"") as first line of request_route.

If I disable topoh everything works perfectly.Topoh is only configured with mask_key only.
Furthermore, not all phones generate this problem, at the moment there are some snoms.

ACK sip:user@phone_ip:2048;line=kkqgarrj SIP/2.0
Via: SIP/2.0/UDP proxy_sip_ip;branch=z9hG4bK2ec.e7126a3134bef9d974f57dd43ebd4ea2.0
Route: <sip:kamailio_ip:5060;lr;received=sip:phone_ip:2048>
Max-Forwards: 66
From: <sip:111111111@sip.example.com>;tag=pDc1BQ0B57Ujj
To: <sip:user@sip.example.com>;tag=vslvowy4y2
Call-ID: 40a6c905-c0c8-4c20-b3f7-397b3fce58b6
CSeq: 90285584 ACK
Content Length: 0

There is no difference between the ACK of the snom and other phones that work.
The only difference I noticed is that the snom 487 contains Contact field in the header.

Any suggestions? Thank you