after hours and hours of testing, installing, contacting support (thanks Oleg ;-) ), i found out that you can expect problems with some brands SD-Cards.
i used Kingston SDCIT/16Gb and SDCIT/32Gb cards with brand new Raspberry Pi Model 3, and have had much troubles with it.
After googling in the internet, i found out these cards are not 100% compatible with RaspBerry Pi.
So, please, take care for the right brand and type (also the type is important) using in RaspBerry Pi.
Customer support service by UserEcho