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.