Thank you very much for the feedback on this.
Unfortunately I don't have the full SIP for that callid, will definitely save it next time when/if it happens again. Looking over the config, indeed there is a `$avp(lastdst)` call in onreply route but there is no "delete" for it throughout the whole config.
However, on that system, we are using tm module with (the new) "callid_cseq_matching" parameter set, which might lead to some transaction mismatching (and maybe to some transaction time-out?!)
I am closing this issue for now and will take this into consideration when/if it happens again.
Thanks, Stefan