Hi Wolfgang,
Thanks for this.
I still can’t reproduce the error, following your steps, so perhaps I’m doing something slightly different to you:
My steps were:

1. Opened a model – The supplied EA Example from Sparx
2. Selected the ‘Modeling Basics’ package
3. Right-click, Extensions, eaDocX, Open in Excel
4. (big spreadsheet opens with lots of elements)
5. closed that spreadsheet – using the ‘X’ button top-right of the spreadsheet, above ‘Help’
6. ‘don’t save’ option
7. Opened the ‘Using the example model’ diagram above
8. Added and Activity to it
9. …all seemed to go OK.

Can you repeat these steps? Make sure the EA Example model is in a place where you’re allowed to do updates to it (that is, move it from Program Files to MyDocuments)

Please let me know what happens – we’ll fix this !!!!