Then:

• Start MSM download tool on computer (to flash stock rom)

• Push and hold the down etc for booting to EDL mode

• (while holding down the buttons) Plugin USB to computer

• (while still holding down the buttons) wait for MSMdownloadtool to recognize the phone

• Start the upload of the stock rom

After this my 6T was restored. Then I enabled the OEM unlocking again and the advanced reboot options. After that I was again able to unlock the phone and this time it didn't get stuck at the 'unlocked bootloader' anymore.

Hope it helps!