This bug in the CR runtimes from CR9 Service Pack 4 was reported by a user:

Here is the story.

I have completely wiped the PC I was using and had it reimaged.
I installed Oracle, MDAC, .NET and CR9 (with cr90win_en_sp3.zip) and then
installed
CRD build from March.

The reports exported to Excel perfectly.

I then installed the CR9 cr90win_en_sp4.zip along with cr90mainwin_en.zip
and cr90dbexwin_en.zip.

This cause the export to Excel to fail by adding merged columns.

I uninstalled CR9 and reinstalled with the sp3 and again the export was ok.

Uninstalled CRD March build and installed the latest build of CRD and the
export again is fine.

So the problem appears to be with CR9 Service Pack 4 or the hot fixes.

Thought this might help you if someone else runs into this issue.