I'm not sure I've ever seen this error message outside the context of duplicate variable names. I'm not sure what the issue is, but looking at the output, it seems that Q112a may have all missing values. Maybe split can't be run on such a variable? (Is it expected that no data was collected for that variable?)
If that's the case about split, then the logic in the do-file will need to change slightly. I'll think more about how to modify that logic. I think a workaround for now would be to remove Q112a from your XLSForm before exporting the survey sheet to CSV. Then the do-file won't recognize Q112a as select_multiple and won't try to split it.