I had this same issue, I got a little to busy to post on the forums and decided that I would remove the Add-In. After removing the Add-In I would get the same message you describe. I know this post is old, but I just was getting this issue earlier today. Since this is the first link in Google Results, I thought I'd add the solution. It's one of those simple Doh solutions but, Go to your add-in options, and turn off this add-in. Here is a detailed instruction list for Excel 2007:
1) Click the Microsoft Button in the top left off the excel window.
2) Go to Excel Options (Located at the Bottom Right of the Menu)
3) Select Add-Ins from the Left Panel
4) At the bottom of the window Next to "Manage:" Make sure the drop down has Excel Add-Ins selected and click the "Go" button.
5) UN-check the HTMLMaker Add-In check box and click "OK"
NOTE: If you uninstalled HTMLMaker previous to these steps Excel will warn you the it could not find the Add-In and ask you if it should remove the add-in from the add-ins list completely, I removed completely.