Nextcloud removes recently synced files on Raspberry Pi 3 after reboot

Hello,
I’ve been using Nextcloud on Raspberry Pi 3 for 3 months, but recently I’ve faced strange problem with syncing files between clients and my Nextcloud home server.I’m using Nextcloud Desktop Client for Ubuntu 16.04 on my laptop and Nextcloud Client App for Android on smartphone. When I create some files locally via desktop client those files sync just perfectly well with Nextcloud and are visible both via desktop client, web client and mobile app , but after I reboot my Raspberry Pi desktop app says that those created files was removed. Also they are not longer visible in desktop client,mobile client or web client (even not stored in trashbin). With direct entering to the data folder via ssh, files are clearly there and aren’t removed. It looks that those files simply no longer visible by Nextcloud at all after reboot. Forced sync doesn’t help either.
For test I’ve created random text file ‘q.txt’ , synced it with NC and reboot server. That’s what I’ve found in my local log file:

#=#=#=#=# Propagation starts 2017-07-11T11:53:37 (last step: 428 msec, total: 428 msec) 08:53:37||q.txt|INST_SYNC|Up|1499763214|bbae03df0b30054d849588cd9a64a41a|26|00005598ocueaw5d9ngb|4||204|0|1499763209|e9aa6e0d209fede5103c6cacec2ca4ac|00005598ocueaw5d9ngb|INST_NONE|
    #=#=#=# Syncrun finished 2017-07-11T11:53:38 (last step: 408 msec, total: 837 msec)
    #=#=#=# Syncrun started 2017-07-11T11:55:19
    #=#=#=#=# Propagation starts 2017-07-11T11:55:21 (last step: 1790 msec, total: 1790 msec)
    ||q.txt|INST_REMOVE|Down|1499763214||26||4||0|0|0|||INST_NONE|
    #=#=#=# Syncrun finished 2017-07-11T11:55:21 (last step: 19 msec, total: 1810 msec)
    #=#=#=# Syncrun started 2017-07-11T11:55:23
    #=#=#=#=# Propagation starts 2017-07-11T11:55:24 (last step: 922 msec, total: 922 msec)
    #=#=#=# Syncrun finished 2017-07-11T11:55:24 (last step: 11 msec, total: 933 msec) 

I’m not sure if this is problem with client or nextcloud server itself. I’m not experienced yet with Nextcloud Administration and Server Administration overall. Need some help.
My system:
Nextcloud 12.0.0 with ACPU cache enabled and Cron for background tasks
PHP 5.6.30
Apache 2.4.10
MariaDB 10.0.30