It seems like you hitting this fatal error :

Fatal error: Call to undefined method JDocumentRaw::addCustomTag() in /xxx/plugins/system/mredirect/mredirect.php on line 36

I already applied some fix in this file JoomlaFolder/plugins/system/mredirect/mredirect.php, it should work fine now.

I would suggest you report this issue from this 3rd party plugins provider.