Hi janis.peter and welcome to the Mattermost forums!
As mentioned in the important upgrade notes, you need to first upgrade to 5.37.10 before you can continue the upgrade to newer versions.
So instead of doing 5.31.7 → 6.x → 7.x, you would do 5.31.7 → 5.37.10 → 7.4.0.
Can you please try that and let me know if this helps with the problems you’re seeing?