I have similar problem, sync of some folders became Out of sync randomly, time to time.
In my situation, I can oppose scenario, mentioned by RomanZ, I have small group of friend, connected for my shared folder, but myself, I have two devices shared as Read and Write and for unknown reason, one of the phone with write permision became Out of sync after some days, even some hours.
Whats strange, I even update files through device, which transfer updated files to the second one and later, it is the first device, which slip out of sync and I have so disconnect it and connect once again as by sharing link made by the second one.
And, of course, since I have apprx. 10 phones in sharing group, I know on weekly basis, some of the deviced became also out of sync (mostly Android 5 ones)
As an IT guy, I have tried every "workaround tricks" I can find out, everytime, to be sure, I manually exit sync and even restart phone to be sure, there are no frozen opened-file-handles.
RomanZ, If you want to track this problem seriously, I can send every possible log and data, you can use. Just as a first one, I am using this only on Android phones/tablets, both phones are rooted, app si up to date, folders are placed on SD card, no computer in group. One write enabled device is Android 4.4, second one is 5.0, both of them stores debug data I can access, I can track some permision problems with terminal..... anything, you can use, but I really want to solve this, because, since I tried syncthings, I realized. not only open source app are the bests, from my point of view, Sync is far more stable, but still / there are problems, that I am not able, even as an IT with basic linux skills, not able to solve.
