This is a little odd but I can get Excel to show an invalid sheet name error after opening a workbook in IE using the webdemo code.
What I do:
1. Run the webdemo.
2. Open the resulting workbook in my browser (I.E.)
3. Once it is open, I rename one of the tabs to include a space in the name. (Space is valid)
4. Click somewhere on the sheet.
5. Go back and rename the tab to take out the space.
6. Click somewhere in the sheet. Now you should get the invalid sheet name dialog.
If I save the sheet from the browser before doing this (after step 2), I don't get this error.
I am using Cells for Java 18.104.22.168 (a fixed one that I got on a previous problem a couple days ago)
Excel 2003 SP2
IE 6.0.2900 SP2