Most, if not all cells, should be formatted as text. The default template spreadsheet shipped with ITSM has this column defined as text. Somewhere during a cut and paste or some other action when populating data here, the format got changed. When entering a number in this cell, you should see the cell with a warning as shown below. If you ensure the cells are formatted as text and appear as below, there should be no further issue with the transformation.
It seems You were right - I double checked the data in excel and I found that some values were formatted like =128+256
After manual re-entering the data import went correctly.