Upgrade from 10.8.6 results in problem loading web resources

I have for the past couple of years been running gitlab-ce. I have sparingly upgraded since nothing was broken, but when version 12 came out I thought it was time to give it a go.
So I upgraded to latest version 10 to 10.8.6. All OK, everything worked just fine.

Then I took the leap to 11.11.3 and problems began. What used to show a well styled webpage when logging in now has flaws. On inspecting the browser-console I find these errors:

I then upgraded to 12.0.1, but problem persists. Everything else works well, but web interfaces is jumbled up.
If I log in, things look even worse.

Tried to google may way to a solution, but I didn’t find anything that seemed relevant.
Any pointer greatly appreciated.