Hi,
You cannot exceed 256 columns in any version of Excel. However, exceeding 256 does not seem to be the actual case in your situation.
On occasion, some character, possibly even a non-printable one, is found in the last cell of a column or row (or both). Go to column IV and delete the column (possibly a few more if necessary). Then, you will be able to insert columns as normal.
Bye,
Jay