MediaWiki doesn't require PHP to have support for mbstring [1] (it has fallback functions to handle that)
The HtmlFormatter class, part of MediaWiki core in MediaWiki 1.22 that's being used only by Mobile Frontend for now, is accessing "mb_" PHP functions that aren't being defined as fallback, giving fatal errors when PHP doesn't have mbstring support.
The missing function is mb_convert_encoding
We should provide a fallback function for it, or check if ( function_exists( 'mb_convert_encoding' ) ) before using it (as it's being done in DatabasePostgres.php line 437
It currently breaks Mobile Frontend on installations without mbstring
[1] https://www.mediawiki.org/wiki/PHP_configuration#Optional
Version: 1.22.0
Severity: normal
URL: https://www.mediawiki.org/wiki/Thread:Project:Support_desk/_Extension_:_%28MobileFrontEnd%29_-_!ERROR!_-_Blank_Page