My guess is that you are a "victim" of FATCA
These investment options are available to locals, who are NOT subject to US taxes
Any Americans living in Germany or Germans living in the US have had these issues recently
I have many friends, who were told by their banks, that they can not longer keep investment accounts, due to the ridiculous reporting requirements imposed by FATCA
There are several banks that still allow US persons (citizens, residents, greencard holders) to have an investment account, but there are few an far in between.
Commerzbank only keeps it for existing customers, as the due diligence for new customers is to cumbersome (but it does not hurt to ask, if you are an existing customer)
Online brokers
https://www.flatex.de/and
https://www.interactivebrokers.com/ind/en/main.phphave been recommended by friends, who needed fo find solutions.
I don't have experience with neither, so take this with a grain of salt.
Generally speaking, aside from the exchange rate risk, you are probably better off to invest directly in the US.
I would research cheaper and more efficient ways to transfer money to the US and invest directly with Vanguard, especially as you are not considering to stay in Germany long term