Though I won this game as black I had the feeling something went the wrong way and that white should have won. Can some of you help me to point out some mistakes (esp. in fuseki since I feel that's where the problem was).

Regarding 87, at least, black may have enough outside strength to kill outright with P18.

How about in H3 direction?

C11 is better as an approach to the corner. I would prefer D15 to work with the right side.
B4 cannot expect C3 so perhaps E6 is better.
C3 seems to contradict E6, so why not jump once more. If W then play C3 you can play bD9, wD8, bG9.
G5 feels funny. Why not H4?
Instrad of K4, try H4 and then play K3 if wG3.
Instead of L4 push at G4 and see how W responds to the cut. You probably then have K3 and a capture of L3.
I'd play F17 instead of J17, but I prefer Q10.
I think i wider pincer a Q10 is preferable to R8.

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Still officially AGA 5d but I play so irregularly these days that I am probably only 3d or 4d over the board (but hopefully still 5d in terms of knowledge, theory and the ability to contribute).

Thx guys for your opinions. I appreciate it and it actually brought me forward, because up to your post I didn't sense g5 as a mistake, but it probably was.