I just hit the same issue, but with a twist:

I've got an ADO recordset that's created from a SQL Server proc. The
recordset returned is based on some passed in criteria. There are 9 fields
in the recordset passed in to CopyFromRecordset. Two of those fields are
defined as varchar(1000). But only ONE of them seems to be causing a problem
and, to further my pain, only SOME of the time. That is, depending on the
criteria passed in, one resultset will throw the error, another won't.
Adjusting the problematic field down to varchar(900) fixes the problem, but I
still have that other field at varchar(1000) that ALWAYS works.

Aaargh. Any ideas on this new angle?
x.