Export file is “not a valid MAL export file”

Yep.

The import/export feature is generally done through xml files, which contains infos from the database that the site will parse to know what to add/delete to your datas. However, every site has a different structure, so unless they go extra lengths to parse all files, the one you get from a site won't work on another one.

You can search for online convertors for the xml from MAL to the site you want, or just manually add your entries; you don't have that much entries in your lists, and the splitting of franchises are not necessarily the same on another site, anyway.