Meanwhile I removed all settings and plugins and started from scratch. So far it looks good. Maybe really a plugin caused that trouble, even I disabled all of them, except the build in ones. Or it is related to the Maxthon upgrade history since I use that browser for a very long time already. As next step I'll start enabling the 4 additional plugins again to verify them.
Edit:
Hmm, while installing the previously saved plugins again I found twice a Maxthon search plugin v1.2.4 but with different size and file name under ...\Maxthon3\Users\guest\Addons Maybe that was the problem.