I have never come across this kind of issue when a program throws up an error message about another program.
Start Outlook 2013 in safe mode, Hold Ctrl and click on outlook icon, check if you get the error message
If you do not see the error message then disable the add-ins in outlook..
Have you installed office 2013 as 32 bit or 64 bit version????
Ok. I followed your advice explicitly. I held down the ctrl key, and clicked the outlook icon on my taskbar, although I didn't actually see any indication that outlook was in fact in safe mode. But I started a new email message, tried to attach an excel file,
and got the exact same error message. Then I closed the new message, without saving it, btw, and when I opened this webpage, the error message popped up again. Appears I wasn't in safe mode the first time. So I did it again, this time the computer asking if
I DID want to start in safe mode, and I did. No difference. Same error. So, then I went to "Options\Add-ins, and made sure all add-ins were disabled, New email message, tried to attach an excel file, same error message. My office installation is 32 bit also.
I've used the un-installer fix-it app from Microsoft, un-installed from the control panel, run the "repair, both "quick" and the more extensive "online" repair, probably 6 or 8 times now. No luck.