Hi Coen,
I don't know if you saw my OML.
I have the actions of exporting the tables in timers, which are scheduled for when you publish. I put a breakpoint in excel (see the image below, and put in the action of importing the sheet TblPQfcrs (BootstrapTblPQfcrs) the table I found empty. I start the debugger and then publish again, just change a widget a little up or down, whatever you need to publish later. At the end of it, these timers will start, and will give you these errors that I showed you.

As you can see from the messages, in certain fields it is waiting for one type of variable and another. To validate if what you have is well done, is in excel, leave only one line per sheet with values, and validate that the types are as expected. And so make sure you have your right logic.
In case the correct logic happens, I suggest that you make validations field by field, to prevent the user to enter incorrect types and have these problems. Basically before assigning the excel line to the structure, validate the values and if not what is expected, either convert the value or discard the line, but this is already a business decision.
If you can't, I can give you more help, but it would be helpful to have your own OML with this part of importing the excel file. But I started by doing that test I suggested, where you put all the sheets, with just one row of data.
Cheers,
Nuno Verdasac