Hey, up till now (lvl 15) I only played co-op vs ai games or custom games.

Since I am lvl 15, I just don't receive any XP/IP anymore after finishing a game, what is the reason for that? Is it capped? I asked in the client chats... but well... nobody answers, hope this isn't the case in the forum community

Yes. There is a cap of how much IP/XP you can gain from playing vs the computer. I don't remember the exact numbers but as you go to higher levels, you will receive less ip/xp. To be honest, it is not a good idea to go until level 15 without playing vs real human cause it will surprise you how more difficult it will be.

Good luck and have fun.

After level 10, the amount of time you can gain ip/xp for playing vs bots has a daily cap, maybe something like 3 hours.

ah, ok, thanks for the info. i think, i am ready for pvp anyways now - imo, the most difficult thing is to get an understanding about the items and the basic mechanics in this game, that's why i chose to play the co-op games. and of course, which champion to use is nothing you can decide by just looking at the art or doing a single game with them

Mastering your skills against bots is one thing, getting roflstomped by real players is another, because they play nothing like the bots.

After lvl 15 you get penalty 25% and timer of how long you can play coop games if time is out you don't get any ip or xp

Hehe, indeed, yesterday I had the time for 2 real games, both wins (thankfully, for upheld motivation ) The first game with Blitzcrank was actually quite decent, had the most kills in the team and was on par in assists and deaths. You have to love the rocket grab.
The second game was with Cassiopeia, my favourite character, and it wasn't pretty at all. Roflstomped doesn't even describe what happened there, I think I had one kill, 10 assists and 9 deaths... we were all pretty bad and struggled the whole game - I have no clue how we won that one in the end - but it was quite fun.