T.38 UDPTL Invalid argument

I am trying to get T.38 Fax to work using Asterisk 16.13.0 (FreeBSD 12.1-RELEASE-p9 amd64) / PJSIP, but when it comes to the point where data is supposed to be sent over UDPTL I see a lot of these messages before it gives up after the maximum amount of retries:

NOTICE[101547][C-0000000a] udptl.c: UDPTL (PJSIP/itsp-3-0000000b): Transmission error to Invalid argument

I can’t see why the underlying sendto() system call would fail with ‘Invalid argument’, no packets are rejected by the firewall, routing table seems in order, no other obvious clues as to why this is happening. Faxing over audio works without problems.

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