When you've installed both MijoSEF and Falang v1.3.1, then you could get the following error:
Fatal error: Call to undefined method FalangManager::getLanguageCode() in /www/htdocs/......./administrator/components/com_mijosef/library/uri.php on line 1600
The problem is that the developer of FaLang has removed that function in the 1.3 version.
As solution, download this zip file, unzip and upload it into the following folder via FTP:
Note: Update FaLang component to 1.3.2