Hi Stoiko,
I upgraded from 5.x to 6.0 a few month ago with given Instructions on:
https://pmg.proxmox.com/wiki/index.php/Upgrade_from_5.x_to_6.0
I didn't compare templates onm this upgrade, no. Everything worked fine until today.
Today I've upgraded from 6.0 to 6.1 and than I had the same...