This guide helped me out last year when UT installation failed the first time and my OPO got stuck: https://forum.xda-developers.com/t/guide-return-your-opo-to-100-stock-current-build-cm13-1-2-znh2kas3p0.2826541/
After that I was able to install UT successful using the UBports installer.

@UBportsNews OTA-13 runs fine on my Oneplus One:-) Thanks a lot for all your work!

@Gentle-Ent Hi, I haven't been around here for some time and it's a bit late now in september, but if you're still looking for a solution, I've had a similar experience with an OPO a few months ago and this helped me out:
https://forum.xda-developers.com/oneplus-one/general/guide-return-opo-to-100-stock-t2826541

After having restored this factory image on the phone I was able to install UT succesfully with the snap version of the Ubports installer.