Residence Permit and Blocked Account Questions for American student

Hello all!
My name is Alyssa and I recently moved to Bremen and am enrolled for one year in language courses. I have been here for about a month and a half now and, being American, I am allowed to stay for up to 90 days without a visa. Naturally, since I am staying longer, I will need a residence permit.
The requirements to make a residence permit application appointment with the Ausländersbehörde is that one brings
- a valid passport
- proof of residence in the city
- a completed application form
- proof of health insurance
- proof of enrollment in courses
- proof of financing (either from someone who will financially support the applicant or through a blocked account.)
I will be proving my finances with a blocked account. There seem to be about three providers that dominate the market - Fintiba, Coracle, and Deutsche Bank. From what I have read, Fintiba, although sometimes the more expensive option, is the most efficient, easy to use and reliable, whereas Deutsche Bank is also quite dependable, but a little bit more cumbersome and slow. I was considering using Fintiba until I started reading that apparently Fintiba will not always work with Americans because of some kind of tax evasion law that the US put in place that makes it hard to work with. I am really not sure what the best option is, and honestly all the information out there is a bit overwhelming... I want to make a wise decision since it is a lot of money I will be entrusting to the chosen bank, but I am also running a bit short on time.
Has anyone else worked with any of these banks/providers- particularly Americans?
Please let me know what your experience was, what the process looked like and anything that might be useful to know!
Thank you!

There is a big issue, which has been covered multiple times on this site about USA persons using foreign financial institutions. What has not been mentioned as far as I know is the context of a blocked account. The IRS has strong-armed a requirement through for foreign banks and financial institutes to annually report the information of American account holders. And Americans should be reporting foreign holdings of over 10,000 USD on a yearly FBAR from independently of their federal tax return as well. 

To open any account in German banks (and most other countries) one has to sign a form stating whether or not they are a “US Person”; which includes not only citizens but also US alien residents. This gives the bank permission to give out the otherwise personal information. They have effectively gone around the German bank secrecy laws. Despite the fact that all banks comply with this, otherwise at the risk of being shut out of the US financial market or face fines which have resulted in billion USD fines on some Swiss banks, many have decided it is too much of a risk and either don't take US person clients or limit them to a basic Giro account. Even having German citizenship in addition, as I do, doesn't change this.

Unless someone else here can give a definite tip, you will simply have to inquire at the various institutes if they will do this for an American. Like with a basis Giro account, such a blocked account might be allowed at some, although other types of accounts or investments aren't.

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