Monca10, have you had success yet? I searched the forums and pretty much the solution is relaunching the LPE launcher.
Something you could try is reboot your computer, bring up the LPE launcher and at the top right where you select the servers, click the drop-down and re-select Official. Then at the bottom left of LPE click Full Scan (should only take a few minutes).
Then try PLAY.
Hope that helps!
Wayne