I have been able to get the logging working (by testing it with some real bad metadatavalues) and it started logging. Unfortunately the scenario I describe below does not result in any logging.
After tons of tries and investigation I finally found a pattern in the results.
For test I am using an existing Excel-file I have used for ages. Its date is old (2014):
Northwind June Expense Categories.xlsx
I dit the following test in importing this and another XLSX file, but all with some metadata-field for the Tijdvak-field.
- Import Northwind June Expense Categories.xlsx and keep its destination name the same.
This will result in no metadata added in SharePoint Online.
- Import Northwind June Expense Categories.xlsx and change its destination name to a1 (without extension).
This resulted in the metadata being added in SharePoint Online.
This gave me the idea to start with an new Excel-document instead of the Northwind one.
- Import Test.xlsx (brand new almost empty) and keep its destination name.
This file is imported correctly with metadata.
So it is something in the Northwind file.
There was a content type id in it (but no property Tijdvak). So lets get rid of the id first by removing this property.
- Import Northwind June Expense Categories.xlsx but now without the contenttype id in it.
Still no metadata.
Ok, then the big cleansing: remove all personal info and xml data in the file.
- Imported the clean Northwind June Expense Categories.xlsx.
Tada! The file is now imported and the metadata is set correctly.
a. There is something in my Excel file that causes problems, but I have no idea what.
b. When imported SharePoint Online interprets this Excel file somehow and refuses to add metadata.
c. when imported with no Excel extension for the destination filename SharePoint will accept the metadata
Well at least I have the cause localized. I am going to import now a lot of data and we will see if the metadata gets added. I hope these files are ok. I'll keep you informed, if you want.