I don't get any response after submitting *104#. In fact, no response to any USSD command. I am in France right now. Could it be the cause? I doubt it because I did got answers being in Israel. If something can go wrong, it will :-(
I will be back in Germany in a couple of days. I can buy another top up voucher, but I'm afraid to get into the same troubles because I don't understand the cause.

"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"
I don't understand this point. My original credit was € 20.01, I checked it. Why it was reduced to € 3 after 1.5 days ? Probably, I don't understand something very basic. I thought that the flatrate XL requires € 19.99 and I can use the Internet for the whole month. Do I miss something ? Could it be that I bought a wrong voucher (e.g. only for the voice SIM, not for the data) ?
Thanks.

If you had € 20.01 left and you ordered the Internet Flatrate XL, your credit would have dropped to € 0.06 in the moment when the fee for the flatrate was deducted. In this moment you would also have been notified of your credit dropping below € 3.
I suspect you have started accessing the web before the Internet Flatrate XL has been activated (you get a SMS notification for that). By this you may have reduced your credit to less than € 19.95 and so the Internet Flatrate XL could not be activated any longer, thus data was billed volume-based in the standard tariff of € 0.24/MB in 400KB-increments (the big increment may be the reason why you were billed more than the actual data volume generated).

The USSD-commands should work on any roaming network. Has MDMA actually shown reception of a roaming network when you tried it? Have you closed Mobile Partner before starting MDMA? If so, there could be some issue on the current roaming network - then try another French network by doing a manual network selection through Mobile Partner (under Tools | Options | Network | Registration Mode | Manual Search and Register).

Sounds more than reasonable, althgough I have defenitely received the SMS message mentioning the Flatrate XL. But, probably, I tried to surf even beforehand. I must confess all this procedure looks unnecessarily complicated. I know I am not a master, but I do have a general computing experince. Making such dramatical mistakes TWICE !!!
This mean something is too confusing. Anyway, while in Germany I will buy another voucher and try again. This time after awaiting the SMS message.
Would you advise to buy the € 30 voucher for not being near the edge (if I would do this the last time, everything was working fine, wasn't it ?).
Thanks, buddy !

You've had really bad luck.

I think you actually get two messages after subscription - the first one after seconds confirming your order and the second one after a couple of minutes confirming activation of the ordered option. So don't start surfing before receiving the second one.

Recharging with € 30 would of course give you a buffer of max 41 MB before the Internet Flatrate XL is being activated, but you can save those € 10 if you're a bit more patient after subscription. ;-)

Which network? Bouygues?

Yeah ! What's wrong with it ?

Inquisitor, please help ! I am in Germany now. I bought another O2 voucer. I topped it up and received the SMS confirmation. I also succeed to see my account via USSD (*101#). It shows Euro 20.04. But, when I am trying *104#, I get the following message:

Ein technischer Fehler ist aufgetreten. Bitte probieren Sie es sp{ter noch einmal.

I tried many times starting from yesterday night and from different locations (Darmstadt, Frankfurt Airport, Frankfurt City). I really got exhausted. Why Tchibo (or O2, or whatever) is so ufriendly to me ?
What is the cause, and what can I do ?
Thanks.

They seem to have some technical problem.

Alternatively try so subscribe to the Internet Flatrate XL by sending a text message with "START DATENFLAT" to 7777.

Sorry for the dilettantism, could you, please, to describe in greater details (as you did for USSD) ? At the moment, I can get connected to the Internet only occasionally. Therefore any Googling is somewhat complicated.
Thanks.

I have sent a text message with "START DATENFLAT" to 7777 using Mobile Partner. No response in more than 1 hour. I am not even sure that the message has been actually sent (no Sent directory to check). I am again in a total fog. USSD is still unavailable for the required option.