🇧🇪Belgium @mchvdm

Ah true, next to removing those folders I also removed the composer.json file that was residing in my root (/web) folder. Maybe that was the solution and not (just) the removing of the extra folders.

I had this issue as well and was able to solve it. Of course you might have an unrelated issue.

In my core folder I had two folders (one was named 'b', the other was called 'core') with inside those folders another core folder.
And in those two core folders there was a folder called 'modules' with a few module folders.

See attached screenshot for clarity.

After deleting both folders ('b' and 'core') the issue was fixed.

I do not know where those folders came from, I did a lot of testing on that installation and it was an upgrade from a D8.