The URLs that Piwik 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. There might be better solutions than disabling it but we don’t know since we are not using it.

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.