Pjsip task processor outreg queue full with TLS

I use pjsip with TLS for my provider. From time to time the register failed and is shown as “Rejected”.
But the cause seems to be the task processor for pjsip/outreg, because the log says:
“WARNING taskprocessor.c: The ‘pjsip/outreg/xxxxxx’ task processor queue reached 500 scheduled tasks again.”

The pjsip debug log shows the normal activity for register. There is no high load on the system (error occurs often in the evening when the system is not used at all).
What can cause the task processor queue for pjsip/outreg to be full with TLS without high load?

A deadlock or issue within the outbound registration support or PJSIP would cause it. What version of Asterisk?

The Asterisk version is 16.13.0.

Then I would suggest filing an issue on the issue tracker[1] with a backtrace[2].

[1] https://issues.asterisk.org/jira
[2] https://wiki.asterisk.org/wiki/display/AST/Getting+a+Backtrace