The error seems to affect particular variables within the dataset(s), even though there's nothing obviously different about these variables compared to any of the others.
If I turn developed mode on the full message is:
Thanks in advance for any help.Debug
Error in private$.readDatasetHeader(self$options$varsRequired): attempt to set index 1/1 in SET_STRING_ELT
self$readDataset(TRUE)
private$.readDatasetHeader(self$options$varsRequired)