If the list is valid and Timmy is in it, check if the previous matrikel is equal to the current one you're checking

It seems that N.E. entries are dropped somewhere during the calculation. Is this check before or after the N.E. entry drop?

The average grade of all the participants to 2 decimal places (N.E. does not count as failed student/participant)

So what grade does N.E. count as? Does it even count as a participant?

F.N.B.: Failed free attempt (counts as 5.0 for grade average purposes)

What about best grade? If there is 5.0 and F.N.B. together, should 5.0 or F.N.B. returned? Similarly, how does N.E. tie with this part?

Expected: 'Error => Matrikel 50621 has an invalid grade: 0.9', instead got: 'Error => Invalid list'

But the description also says

make sures there's always a pair of: 5 digits and a grade (1.0-4.0, 5.0, N.E., F.N.B.). Lists always start with a matrikel number and then a grade, anything else is invalid.

check if the list is valid with all its variations, if not return this error: Error => Invalid list

So 50621 0.9 should be rejected as invalid list, since 0.9 is not a grade according to the definition of a grade. Why is it invalid grade?

More importantly, what is a valid list in the first place? This is not even defined in the description.

juliankolbe, maybe you could make another kata
where you revenge the King of Woodlands through attacking
his kingdom.

This is just the unlimited knapsack problem, which means it's a duplicate to many existing katas.

The errors return order in the kata description is not clear enough.

You said in the description:

You need to check for the errors in order of appearance and return the first one caught. 
....
....
So if a list has several errors, such as "Invalid list" and "Double matrikel number entry", "Error => Invalid list" should be returned

In fact, I finally got the right order by testing and guessing:

The highest priority level is Invalid list, the second priority is not found, only invalid grade and Double matrikel number are depending on the appearance of the sequence. am I right?