Joomla allows templates to define its own function renderMessage() to generate html snippet for messages area in any suitable way, and our mobile templates do it. But in your case you use Joomla Template Framework plugin that defines that function prior to any template is loaded that results in above issue. The problem is that Joomla Template Framework is system plugin (i.e. it is loaded at Joomla initialization stage) and MobileJoomla cannot disable it (plugins of other types can be disabled).
From source of the Joomla Template Framework plugin it looks like renderMessage definition might be skipped if MobileJoomla plugin is loaded before Joomla Template Framework plugin. So , could you open Mobile Joomla! system plugin settings and check that its ordering (-99 by default) is less than ordering of the Joomla Template Framework plugin (-5000 by default), that is most likely one of them should be changed appropriately?