An explanation not yet mentioned is u beeing kicked due to mods. Resently, especially since 4.09 there have been an bombastic increase in random kicks.
My guess is 4.09 dont like mods
Seriously though, iv been kicked for no reason more times in the last 6 months than previouse 7 years put together. (not counting lag issues etc of course)
3-4 kicks a night isnt uncommen, luckely i dont fly much anymore
Edit: Saw that u said u had no mods, but im uncleare if server uses them.