"First time" file upload failing

Experiencing a bug when uploading a file for the first time on an EpiServer page.

Using the "+"-button as the image shows:

I get the following result with the "Content type "DocumentFile" is not allowed..."-message when hovering on "Mislyktes" (Norwegian for "Failed"):

The "DocumentFile" is defined as follows:

Getting this console message if helpful:

If I, however, use drag and drop to add the first file - it works. And when I then add a new file with the "+"-button after a file is uploaded with drag and drop, this will now also work.

So, the issue only occurs when uploading a file with the "+"-button the first time.

Is this a known issue? Or does anyone have an idea what causes this behavior?

Running EpiServer 11.12