it seems that there is a strong bug after the joomla 3.4.0 and the T3 2.4.5 upgrades.
This is the problem:
if you work on a joomla menu or on the template management or on the megamenu configuration and then you save your work, the ENTIRE megamenu configuration RETURNS TO THE DEFAULT, all the previous configuration work is lost!!!
Please investigate about this, it is a very frustrating problem.