This is discussed at the Microsoft Knowledge Base, article 213904:
http://support.microsoft.com/default.aspx?scid=kb;en-us;213904
The error occurs when the workbook contains more than approximately
4,000 different combinations of cell formats. For example when you
format cell A1 as currency, then cell C1 as currency, Excel tracks
that as two ranges of formats. I used to encounter this error when I
was required to use a particular workbook at work: I got around the
problem by changing the format of some of the of "back office" tabs of
the workbook to a generic format. So if you have a lot of data tables
holding VLOOKUP information, for instance, but are not presented to
end users or for which appearance is not critical, you can do this:
1. Open the "too many formats" workbook, and simultaneously open a
blank workbook
2. In the blank workbook, select a cell and (from the menu) click
3. Flip to a non-critical tab of the affected workbook, right-click on
the "select all cells" in the upper left corner and in the menu that
appears, click >Paste Special >Formats. This converts all formatted
regions into unformatted space.
Dave O
Eschew Obfuscation