May I suggest that there is something wrong with the upload procedure on the server. After multiple failures -
I created a item manually on ERPnext. Then I exported it to an .xlsx file. Deleted the items in ERPnext and uploaded the exported file straight from my Download folder without opening it. - Failed!
Uploaded the artificial feet in different sizes and this time ERPnext accepted the items. But only the variants, not the template the variants belong to. How is that possible?