please have a look at the following:
In:= Subscript[\[Sigma], Subscript[K, Euler]] = 10
In:= \[Sigma] = 5
In:= Subscript[\[Sigma], Subscript[K, Euler]]/\[Sigma]
Out= Subscript[5, Subscript[K, Euler]]/5
As a photo too, because i think it is better visible then:
Does sombody have an idea on how to stop this problem and why it occurs. For me it is kind of mandatory to use those index.
As you've noticed there are odd things that can happen using subscripts (and superscripts) in calculations. The solution: Don't use subscript and superscripts in calculations.
But you can have the desired effect (which was posted recently either in this forum or Mathematica.StackExchange - I can't find it at the moment) using the Format statement.
Format[\[Sigma]KEuler, Subscript[\[Sigma], Subscript[K, Euler]]];
\[Sigma]KEuler = 10;
\[Sigma] = 5;
(* 2 *)
This support article addresses the issue of variables with subscripts and superscripts: https://support.wolfram.com/43606
The issue can also be fixed when naming Sigma into Sigma_Something
But an underscore isn't a legitimate part of a variable name.