It might be unpredictable when the /tmp is full.
We are investigating this issue since we heard about that and many thanks to @yorik for including the output from the serial cable! It appears that something slips to the stable release of kernel 4.4 LTS. In some cases, the router rebooted a few times, but after that, it works flawlessly and on the other hand, there might be a nasty bug, which might do reboot loop and we are really sorry for that!
As some of you can notice in the Czech section, we are working on Turris OS 3.11.10, where it should be solved. The RC version should be released tomorrow.