Thank you tshbgr and warpitaly for reporting your problems too.
Let's wait and official patch/update.
From my side, I cancelled the update and stood at version 7.2 for the moment.
Have a great day
After migrating Znuny from v7.2 to 7.3.1 noticed the following error in ticket header display.
Seems it's a CSS problem.
I installed the upgrade twice each time from a 7.2 snapshot. Same display error.