Excel wraps any numeric values into a non-string cells by default. Importing such content fails then on the exception attached so one needs to encode all these cells with a single quote beforehand, in fact they become strings.

Similarly if you have a property of just a numeric value that is a subject of export, Excel is not able to read it properly at the moment, unless it's converted to a string cell.

The module can easily be more content editors friendly. Alternative XLS editors, like LibreOffice, used to be able to cope with it as they cast to string implicitly.