Custom variables with proxy-hide-piwik-url

I am using Piwik 2.4.1 and the newest version of matomo/misc/proxy-hide-piwik-url at master · matomo-org/matomo · GitHub

Everything goes fine to the point where I need to set custom variables. Any call to the proxy file with custom variable ends with 404 error. The console shows this error for the call to proxy file with all the variables. Error 404 is on line 19 of the script generated by piwik.php.

When I delete the _cvar variable from the URL it doesn’t respond with the 404. Please see example variable:

I tried to enter the URL manually and it returns 404 error page on my website. When I delete _cvar variable from the URL the GIF image is provided without any errors.

When I call the piwik.php on piwik server with _cvar also GIF image is provided without any errors.

Do you have any solutions for such issue?

Maybe your server is configured to use some kind of security extension that blocks this request by mistake? see for example: How do I configure Piwik when mod_security (or CA SiteMinder) is enabled? - Analytics Platform - Matomo