@Pedro and @Lana

Thank you very much for your help. I understand what the difference between these two sentence are now.

Hi Anja! I totally agree with Lana's explanation.

Spanish learners have to deal with this chalenge: when to use "ser" and when "estar".

In your sentence you have to choose "estás muy loco" because that's something we normally say colloquially to a friend in a positiv/friendly/funny way.

If we say "tú eres un loco", then we are not being friendly at all :)

If we Spanish speakers hear "tú eres loco" from a non native speaker, we inmediatelly understand that he means "tú estás loco". This form without the article is in this case not common for a native.

Also, ich vermute, der Unterschied auf Deutsch hängt davon ab, ob wir ein Substantiv oder ein Adjektiv verwenden : du bist ein Verrückter (eres) oder du bist verrückt (estás) .... ist das wirklich so?? Ich bin nicht sicher :))

Eres loco - you are a crazy person, it’s your permanent quality.

Estás loco - you’re being crazy, you’re acting crazy in that given moment.

Hope that helps :)