This one gave me a bit of a headache 🙂
Firstly, to quickly address “So it seems that i do need to define extra specific columns as [ID] columns, but i do not know which ones.”: The [ID] is something you pick in Excel. It can be any property from your data item with unique values and you can always have at most one column marked as [ID] in your Excel-file. You can’t do anything wrong there (as long as you pick one column with unique values).
Now, regarding the Excel-file you imported. I retrieved it so I could have a look at it and some really weird stuff is going on in there. For some reason, there is a lot more in there than I’m seeing and I have no clue why (maybe you know). On the Product-tab, columns A through K are filled with data you entered. However, even though I don’t see it, Excel also seems to think there is something in columns L through R as well, which would make sense with the error in Triggre from you last screenshot. The same goes for a lot of the rows. When I deleted columns L through R and most of the rows and tried to import the file (on my test environment), it imported just fine. So that would solve the immediate problem I think.
I’ve never seen this before. We will need to build some logic that ignores data that isn’t there, but at the same time Excel seems to think is there.