I tried it on Excel 2010 on an Existing file but without success.
I tried also on Excel 2007, and it succeeded only a few times. Perhaps it is related to Excel propeties, but I didn't find which configuration to change.
In "Format cell> Protection" is unlocked.
In Properties (right click on Excel file)> Security, all users have Full control on this file.
Sometimes, in some attempts of this test, Text sent to another Excel file with gibrish name.
Is it possible that if text is sent form output of previous function in flow, this is causing the problem? (I checked in this case that the text sent is really String). I have not found yet a solution for that.