APs randomly resetting despite config "up to date"

Hi Markus, 

The L2UPFrame is something the AP will signal whenever a client roams to another AP.  Normally you should see one single log appear when the client traverse from AP A to B, logged by A.

From your previous snippet of logs, most L2UPFrame logs look quite normal except for this HTC client "00:ee:bd:8f:06:07" which I find it strange is that while roaming it always trigger the log twice, on seperate APs, this can only be triggered if the client sends a leaving BSS event to the AP.
Perhaps this one has something to do with it's own chipset, if you know the owner of that client , I suggest you can inform to upgrade it's firmware in case HTC has any fixes on the wifi function.

If you have other logs to inspect, maybe I can take a look.

Is the Wifi coverage overlapping in those areas?