It's sounding more and more like T-Mobil DE and T-Mobil IT have both shifted to something new. I'm curious (being a T-Mobil US user), if you log into your account does it now indicate the device you're using as N900? Mine still is showing "unknown device" and no icon. If yours is now showing N900 it may be that T-Mobil in your area has finally updated it's database to include IMEI values for the N900.

I had this issue with AT&T US shortly after fMMS was setup and working. They updated their computers to know N900s and stopped sending MMS to them since technically the N900 doesn't support MMS. In AT&T's case, it would send a standard SMS instructing you on how to log into your AT&T online account and see/read your MMS online. After several calls, over a week, I got them to set the N900 up as a "dual mode" device. That means it would assume you had no MMS until you sent one. (I told them MMS support was released in a "firmware upgrade"...)

It may be you need to now do the same with your T-Mobil provider. I'm hopeful that T-Mobil USA is slow to update their info, or don't bother updating "old" Nokia devices in their database... That way I don't have to deal with this a second time.