SIP Authorization process stops in the middle

asterisk 15.2.0 and 15.5.0
ubuntu 16.04
pjsip build-in

The authorized SIP registering is made of four steps:

  1. REGISTER request w/o Authorization header
  2. UNAUTHORIZED response w/ WWW-Authenticate header
  3. REGISTER request w/ Authorization header
  4. OK response

My problem is that since a few days now the process is not followed entirely. While it was working for several months, it stops working for some unknown reason. Upgrading to asterisk 15.5.0 doesn’t help.

What currently happens is: Asterisk sends request (1), and the provider answers with response (2). However, instead of asterisk now sending request (3), it logs
… res_pjsip_outbound_registration.c:796 schedule_retry: No response received from ‘’ on registration attempt to ‘sip:*****

It does so although wireshark and even pjsip’s own debug output clearly show that response (2) is actually arriving! Then asterisk retries for the configured amount of times, and finally gives up.

The WWW-Authenticate line from response (2) looks okay for me, although I’m not sure about the “stale=true” option at this point in the process. Well, catches from earlier and - more interesting - working examples show that option, too, so I’m not too uncertain about it. However, here it is:
WWW-Authenticate : Digest realm=“”,nonce="*****",stale=true,algorithm=MD5,qop=“auth”

So, what’s going on here? Why does that discrepancy between what arrives and what is written to the logs occur?