I tried to reproduce it on my systems (Vanilla 2.0.18.2, .4 and .8) and the email was sent correctly. What plugins and themes are you using? The issue may depend on them.
Try to write in html <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><body><p>Antrag zum zurücksetzen des Passworts.</p></body></html> sollte gehen :) viel glück