I'm having the same issue.
Coming from LineageOS 16, I tried the snap installer without success. It kept complaining that the connection was lost and I should check the cables. Despite that, it did change the system because I was left with a boot loop showing the UBports logo. By pressing volume down + power I can get to UBports recovery, although I'm not sure what to do with that. I thought of trying to push the UBports zip to the mobile and use the recovery option to install from it.
A bit like you, the installation was successful when I used the deb installer, probably due to snap permissions. Despite the message that it was successful, the boot loop goes on.
Follow up: I was able to successfully install the system. Following a tip from the Telegram group, I flashed TWRP and wiped cache, dalvik, etc. After that, I installed UT using the AppImage installer and could start using the system.