By any chance is the table a SQL Server table rather than native Access? And
if so, do you have any bit fields linked to a checkbox on the form? This
would happen if the original Access form had a Y/N field. If so, SQL Server
has the ability to have a bit field have a NULL value in addition to the 1
and 0 for True/False, and Access has a problem with that - you get the
somewhat misleading (ahem) error message that your report. The cure is to
make sure that all bit-fields do not allow NULL, and have an appropriate
default value in the SQL Server definition. If you used the upsizeing
wizard, it did NOT do this for you. You will also have to run a query to set
the fields that are NULL to 0, as that won't happen for existing records when
you change the definition in SQL Server.
A dead give-away that this is the case is that the checkbox will be 'grayed'
if it is bound to a NULL value in SQL Server, where it will be clear (white)
for a true FALSE and have the X if bound to a TRUE.