This was a weird one. Somehow tab characters got into the udf_value fields in the tag_udf_values table. This caused extra tabs to be entered in to the grid. Since the tab character is used to delineate the grid columns, this was throwing the columns off, so a column with alphanumeric characters was being read where a numeric value was expected.
Execute this sql statement to fix the problem on their system:
update "tag_udf_values" set udf_value = replace(udf_value, char(9), '');
They will be able to reserve the tags to the order now.