Players cannot join to server

When I start a server Players cannot join.. And the server itself is under heavy load ... 54% of the cpu are used without any player and I need more then 500 mb ram to start the server.. After I started the server player seem to join the server but there is NO other communication with the server except this "joining".. After someone "joined" the server skips alot of frames.. hmm better see the server logs... (its not working in online mode or offline mode.. It worked in 1.7 with the same setting and a CPU use of 20% when 2 player were on it)