i have made my own client for the Minecraft Version 1.8.8. I can play normaly in Singelplayer but i cant connect to any server. It always says "cant connect to server" but when i change to a other client, for example the standard 1.8.8, i can join servers again. Any tips?
Are you running the game from within MCP or the launcher? MCP doesn't log you in so you don't have a valid session ID, which doesn't matter for singleplayer (you will have a random username/UUID, but changing it to yours won't fix multiplayer, only singleplayer achievements/statistics). Otherwise, if you made any changes to networking or anything that needs to access the server from the client (note that singleplayer internally runs on a server) you'll only be able to connect to a server with the same mods (I don't know the full details of client-server compatibility since I've never attempted to provide multiplayer support in my own mods, though I do know that ones that only alter server-side code (these can be used in singleplayer or a dedicated server) are compatible with vanilla clients; likewise, a client-side only mod should be compatible with vanilla servers).
Im running the client from the launcher and when im running it, im logged in with my Account so i have my username/UUID. I think that i didnt change anything from Minecrafts code, I only added new things to the gui so my client doesent have anything new in it. But isn't it the case that even if you have mods that the server doesn't have, the description of the server (I think it's called MOTD or something) is still displayed? That is not the case with me either. (Btw. sorry for my late reply).