LeTV X800+ Contacts synchronization problem

Hello,
recently I have the following contact synchronization problem.
LeTV X800+, EUU 5.5.013S, China factory ROM, non-rooted phone.
Problem description: In the "Contacts" application, a big part of my contacts have been deleted (invisible) by the phone itself. After manual export of Google Contacts via VCF type of file and importing manually this VCF file via the phone's Contacts application, all contacts have been "visible" again. Within 24 hours again big part of my contacts have become "invisible" again. Again manual loading of the contacts from the VCF file into Contacts app. Again contacts deletion within 24 hours. VERY, VERY ANNOYING !!!

Any solution?
Thanks in advance!

Calendar synchronization problem

Hello,
I have some issues with calendar synchronization. When I go to settings/accounts/Google I have checked calendar sync but unlike other elements even when untick and tick again, spinning arrows appear, spin few seconds and then stop spinning but don't disappear. Other syncing elements work fine, spinning arrows disappear. I have no exclamation mark saying there is a syncing problem.
Cleares cahes of all possible apps. Even installed google calendar to eliminate stock calendar issue. Reset didn't help.

Calendar data is synchronizing light fast on one device. If I create or delete an event on a phone it appears on widget instantly, but not on the other device. I have to manually force sync on other to get it show