The URLs that Matomo Mobile requests are usually not longer than 500 characters and should not cause any troubles. In the past we noticed some troubles if a WordPress plugin like “iThemes Security” was used though. It should work if you disable this plugin. Or you could try to go to iTheme Security options > System Tweaks > and just disable “Filter long URLS” option.

In case you are not using plugins like iTheme Security you might want to have a look at LimitRequestFieldSize and LimitRequestLine for Apache or large_client_header_buffers for Nginx.