Do you have any memo fields? I was able to reproduce the error using a simple table with a memo field like below. I don't remember the limits for the unicode driver.

Table1
ID Autonumber
Name Text 50
Notes Memo

Datasource
Microsoft Access with Unicode
CLOB (Checked)
Long Text Buffer (64000)

<cfset form.name = "Alan">
<cfset form.notes = "">
<cfloop from="1" to="640" index="x">
<cfset form.notes = listAppend(form.notes, "0123456789")>
</cfloop>

<cfoutput>#Len(form.notes)#</cfoutput>
<cfinsert datasource="AccessUnicodeDSN" tablename="Table1" formfields="Name,Notes">