I realized that I never read the man page till the end. Thanks to the guys of bytemine at Kieler Linuxtage for answering the question on how to remove files like “-a” which could be interpreted as an option. You just add “–” between the command and the filename.
man rm(1) on Ubuntu also gives another possibility: “rm ./-a”. Wow, am I stupid or what? But finnally I got a solution that I guess i can remember well.