Migration form Elements to Lightroom Classic failing

I tried to import an elements 2021 catalogue to Lightroom Classic (new install and fully updated).

The catalogue works fine in Elements and I could also create a backup in Elements without any errors.

When importing I only get a small fraction of the photos, the rest are not imported: "the files are not available or they are damaged" (translated from Swedish). 

Before doing the import I optimized the catalog and then made a backup, as mentioned, without any errors.

Any idea what I can do?