The solution to the 3G Watcher problem
 

The problem was caused by the TRU-Install program in my modem.
The solution is described here:

Why does Watcher display "Device in software installation mode"?

and here:

How to disable TRU-Install?

Works as a charm :)

What USSD commands in MDMA ?
 

What are the USSD commands to activate "Internet Flatrate XL" by using MDMA ?

Are they the same as in the branded Tchibo Mobile Partner ?

dial *104#
press 2 for "Option buchen"
press 5 for "weitere"
again, press 5 for "weitere"
press 2 for "Internet Flatrate XL"
press 1 to confirm

Afaik Mobile Partner (no matter if it's the generic or branded version) doesn't support USSD at all. However you can also order the "Internet Flatrate XL" by sending a text message with "START DATENFLAT" to 7777 instead of using the USSD-menu.

I thought it was your thread here Germany - Tchibo - Prepaid Wireless Internet Access, from which I copied the above USSD commands.
Anyway, returning to my question is it possible to activate "Internet Flatrate XL" by using USSD commands in MDMA (if yes, what are the commands) ?
Regarding your suggested text message, I must to confess that I have no idea of how to send it. Sorry, you only tought me to send USSD commands in MDMA and to receive SMS by using Mobile Partner. Is sending text message just the other way around ? Unfortunately, I can't experiment by myself as my account is still empty and I can't therefore activate it.

Yes, I'm the author of that wiki page.
I described the USSD-based order process in detail on the first page of this thread.
Just start Mobile Partner, click the "Text" button and below you'll see a new row of symbols, where the frist one (a blue enevlope) stands for creating a new text message. Hit that, enter phone number (7777) and text (START DATENFLAT) and finally click "Send" to submit it.

Thanks ! Two options are always better.

Hi from Germany. I have succesfully engaged Flatrate XL and surfed (though quite slow to my taste) for the last two days. Right now I received this message:
Tchibo mobil: Ihr Guthaben ist unter 3 EUR gefallen. Laden Sie jetzt einfach und bequem online oder per SMS auf. Alle Infos auf www.tchibo-mobil.de
Time: 12/10/2010 13:00:15
Does it mean I am approaching the 5GB limit ? Strange, isn't it ?
Pls, help and advice what to do ?

No, this text message just notifies you of your balance having dropped below € 3. You won't get any notification when reaching 5GB, but you can check the total data volume generated so far through the USSD menu.

The total amount was around 40 MB. So, this is not the problem. The problem is obviously with the wrong flatrate I got. Probably, instead of XL I somehow got to a per hour rate (if there is some). I don't know, what happenned. But it is clear I should not get a message about 3 Euro left (now it is complitely disconnected with 0.02 Euro left), if it would be the XL flatrate. Am I right ? Even after 5 GB (and I have used a small fraction of it) I should have continued with the ISDN speed. Isn't it ?
Anyway, how can I find out what was wrong? I did EVERYTHING exactly according to your instructions and received exactly those messages as you wrote. Is there any way to find out (besides calling Tchibo), what flatrate I have used.
Thanks.

P.S. I can only occasionally get connected to the Internet during the next 5 days.

Have you actually gotten the message confirming the Internet Flatrate XL has been activated? Sometimes it takes a couple of minutes or even hours until it is active.

You would get the notification of your credit dropping below € 3 even with the Internet Flatrate XL if subscription has reduced your credit to below € 3. But of course you shouldn't be billed for any data transferred as soon as the Internet Flatrate XL is active.

Correct. After exceeding 5GB bandwidth is being throttled, but there won't be any additional charges.

Yes, after submitting *104# for the options menu, send 1 for Optionsstatus, which should list you all options currently subscribed.