I have an update, after reading Squid documentation (again and again):
"...being UDP this module may drop packets when the network is under load or congested."
So we changed the Squid log output from UDP to TCP and now there is only 1 instead of 5 up to 20 messages in a single entry. As an example of the load: we have around 15.000 - 20.000 entries per 5 minutes.
But:
The "[processor.javascript] extract_page" error still exists.
It writes an error message per log entry, so around 15-20k error messages in syslog per 5 minutes.
Did you mean this error for the Github issue? Because I couldn't find a single entry regarding this error, no matter which search engine I use.