Just to point out that this has started and runs until Monday 23rd. The instructions are in German but most of the puzzles are obvious (although I can't seem to copy the text into Google Translate). For any non-standard ones, I THINK:
2. Blackout Domino - the blacked out squares have to be isolated - I think they can touch diagonally
6. Fischpaare Knapp Deneben - pairs of fish touching diagonally which don't touch, even diagonally, with other pairs of fish. And just for fun, the clues for the number of fish in a row/column are wrong by 1. I don't know if a 1 clue can be 0.
15 Orthogonales Quadrat - squares should be filled with A1,... A4, B1,..., B4 to D1 to D4 so that no number or letter repeats in a row or column. No repetition of a letter-number combo
16 Pentominoeinbau mit Behurung - right, from my limited German, the clues are Tapa clues, place the pentominoes that are given so that they don't touch orthogonally but they are linked diagonally
17 Doppelblock with four squares
18 Rundweg... Slitherlink but clues inside the loop give the number of corners used, not the number of edges. Clues outside the loop give edges as normal
19 Schlangenest - fill in the letters to make the words. Grey cells show the start of a word and the direction that it goes in.
20 Trinarzerlegung - looks like finding the number in base three from 1 to 202. Not the answer submission involves convertin back to base ten
22 Vier Wind - Four winds but with letter for clues - same letter=same clue, different letters = different clues
24 X - think this is the one where the clue indicates the sum of the first n cells in that direction where n is the first cell. So in the example, 14 = sum of the first 4 and 6 is the sum of the first two.
26 Zeltlager mit Doppelstern - I think it's just tents as normal and then put two stars in each row and column, not touching diagonally, a la Star Battle.
Note: There is every possibility that I've missed a subtle rule but I do hope this helps.