Strange behaviour of pbx 13 vs pbx 12

Hello, colleagues!
Just 2-3 month ago I moved from freepbx 12 (FreePBX to freepbx 13 (v., on Free Asterisk Phone Station v. 13.14.0). Everything was just fine untill recently.
During those days I have been upgrading the new station several times so now its uptodate I would say…
But a couple of weeks ago it started to loose main incoming trunk (we have 3 - 2 of local providers and 1 - by sipnet).
2 trunks are just fine (one - by local and another - by sipnet) - no problems at all, but that third one still gives me hard times.
It just falls with the “registration request sent” repeating message (every 20 sec with every 20 sec retrys),and after some time mistiriously runs up with successful registration. No logs fix this!! They are just as normal as usual, like nothing happening ((
For the same time I tryed to run freepbx 12 with the same registration the same trunk supplier and it registers and runs perfectly without any problems.
I realize that more figures are needed so I would expect advices what should i do.
Of’course i have some answers from the trunk supplier which is here:
“Судя по дампу Астериск какое-то время не отвечает на сообщения SIP 401 Unauthorized со стороны РТУ, точнее отвечает но без тега Authorization. Причины необходимо выяснять на стороне Астериска. (Looking through the damp file from your Asterisk connection you pbx doesnt reply on SIP 401 Unauthorized message from our side, to be more precise - it answers without Authorization tag. The reason of it should be explored in client Asterisk pbx)”.
So the question is why pbx13 runs so badly vs pbx12 with the same configs? and should I roll back to pbx12?
Thank you