Hi Bjarke,
Thank you for your message.
Yes, I just realized today that there was some error in the script for liftover (since GTEx v8 was only provided in hg38 but FUMA is hg19).
I'm sorry about that!!
I'm fixing this right now, and will update online asap.
Should be able to do that by tomorrow latest and I will update you once it's done.
Best,
Kyoko