I installed Windows 7 on an SSD and upgraded it to Windows 10. Then I installed Linux mint 17.2 Cinnamon and had the following partitions:

The boot menu was showing Linux Mint and Windows 10 and I thought everything was fine. UEFI boot configuration showed "ubuntu". However after booting Windows and then rebooting, grub was gone, and in boot configuration there was only "Windows Boot Manager" available.
When I repaired grub2 with grub-install and grub-update I was able boot Linux Mint again, but only as long as I don't boot into Windows 10, which seems to wipe out grub like this every time.

Secureboot and Fastboot are disabled.

/boot/efi contains folders Boot, Microsoft and ubuntu.

Did I do something wrong? How can I get grub2 working permanently?

I found the problem. Looking at the NVRAM with sudo efibootmgr I noticed that the Windows boot loader somehow seems to have the urge to be the first entry in the boot order. When I changed it to grub2 being first, windows overwrites entry 0000 and changes the boot order, even if grub2 was 0000 before, therefore overwriting it.

The solution was setting the Windows boot manager entry inactive but leave it in first position of the boot order:

sudo efibootmgr --bootnum 0000 --inactive
sudo efibootmgr --bootorder 0000,0002,000C,000D

(with 0002 being grub2)

I face exactly same problem to yours.
And here is my solution.

PS. My computer is ASUS and my first os is Win10 the second is ubuntu.

First please make sure Win10 completely shutdown by turnoff the quick startup of Win10. Prevent the issue that Win10 might not completely shutdown.
https://www.tenforums.com/tutorials/4189-turn-off-fast-startup-windows-10-a.html

I found that there is a solution use the software 'boot-repair'.
https://help.ubuntu.com/community/Boot-Repair

So I use the boot usb, by which I could access the ubuntu os environment.

Then open the terminal, and go thru the commands below.

 sudo add-apt-repository ppa:yannubuntu/boot-repair
 sudo apt-get update
 sudo apt-get install -y boot-repair && boot-repair

After the process completed, reboot the computer.

Use special key to go into BootMenu. In my case (ASUS desktop) F8.
http://www.disk-image.com/faq-bootmenu.htm

Do not go into the BIOS/UEFI setting mode!!

(I tried many times if I get into the BIOS/UEFI mode to change the boot sequence it will not work. I can see the Ubuntu option appear again, however after i put it to the first option and restart, it directly go to the Win10. I double check from the BIOS/UEFI mode, it simply make the Ubuntu option disappear again...)

Change the boot option sequence to what U expect.(Ubuntu to first)
Then the problem solved.

Some BIOS/EFI will by default point to EFIMicrosoftBootbootmgfw.efi .
Just move EFIMicrosoftBootbootmgfw.efi and EFIMicrosoftBootBCD to EFIMicrosoftwin10 , or just move the whole folder and toast the EFI entry using bootice or something similar and it will use the fallback efi loader EFIBootBOOTX64.EFI .