I am an American citizen living in Germany with an Aufenthaltstitel (Residency Permit), which is pasted in my American passport. The permit was issued on January 13, 2017 and expired on April 15, 2017. I went today (April 19) to try to renew my permit, which takes two weeks time after filling out the necessary forms. The people at the city offices told me I am not allowed to travel outside of Germany until my residency permit is renewed and valid. I want to travel to Spain (Palma de Mallorca) on April 21 and back on April 23.
Why am I not allowed to travel outside of Germany (even within the EU and Schengen) with an expired Aufenthaltstitel? Why can I not travel simply as an American passport holder while it is expired and being renewed? What parties would have a problem with this kind of travel (i.e. the airline, customs and border control, etc.)?