This error is very general in nature and can occur for a number of reasons.
In one case it may happen when the education class of field is mapped to a value that is not a pure number e.g. in the Graduway file there is a field that may say "Class of 2007". Importacular and indeed Raiser's Edge is expecting a value of simply "2007". Because of this there is an error when this field is mapped. This is irrespective of whether class of is used to look up constituents or not.
The errors may look as follows:
or
In later versions of Importacular there may be a reference to the value mapped in the class of field.
This issue has been fixed as of version 3.1