Hey guys, I finally think I understand what happens. It's very strange, but it works. The idea came because the support told me regarding another issue (the Windows Explorer Crash) that I have to "verify" my installation. Sorry if it sounds more complicated how I explain it, but my English is not perfect.... (as you might know already *ggg*). Well, let's start:
If Impulse doesen't recognize the installation properly (check this with the right mouse click on the app in impulse and choose "verify installation") Impulse would *really* like to download (parts of the program) again in order to update the installation.
The problem is, that this kind of update is hidden and does not appear in the "Update" tab but is shown only with a right mouse click on the application (WindowBlinds) in the "My Software" tab.
So when trying in such a situation to update with a "real" update available in the "Update tab" impulse is internally still waiting for a "correction update" of the incorrect installation (the one which is only available with the right mouse click). Of course then the file size of the "real" update never matches the expected file size of this internally "correction update" Impulse is waiting for.
I had to to this "correction update" twice until Impulse was satisfied, first it was about 20.81 MB and about 800 files. After a reboot I verified the installation again and still it stated that it's incorrect.
So I did it again and this time the download only was 3.09 MB and 408 files. After another reboot now it says that my installation is correct.
With that I finally could update the 7.01 Beta which turned out to be only 4.11MB and 410 files. At the end the update worked without any problem.
What I think is that #1) Impulse is very intransparent in order to know what's really going on and #2) it's a strange thing with this "correction updates" because these are not availabe in the "Update tab" and I reckon no average user has a chance to find out that Impulse refuses regular updates (especially not with this "file downloaded is larger than expected" error) until this correction updates are done.
But although I spent so much time with all this stuff on account of my family I'm finally happy that at least I could install the update now. Let's see if with the 7.01 Beta my permanent Explorer crashes will stop....