Importing Pencil & Paper data - "Input string was not in a correct format"

I have an LS study ("study A", LS 9.12.1) with several CBC and Maxdiff components.

I want to import data from an equivalent study (same CBC and Maxdiff designs etc), called ("study B")

In Study A I used Field | Create Accumulated Data Template File.

I took data exported from Study B, and (I think) edited it to match the template:
var names matching (including case),
variables not listed in the template deleted
values of sys_RespNum changed to avoid matching values in Study A.

Missing values are empty cells in the CSV file.

From Study A I then used Click Field | Accumulate Paper & Pencil Data... and browsed to the .csv file containing my Study B data and…it failed.

The error message was "Input string was not in a correct format"

I have eyeballed the 103 var names and done other checks but I can not see the problem in var names or data.

Any tips on common causes of "Input string was not in a correct format"?

Thanks
Dan