I have seen that many ppl have this issue were the client sometimes says UNDEFINED server and the only fix is to restart your pc.
I had that issue more than twice and after some research i found a post in a forum with a fix for the problem and it worked for me so far without the need to restart my pc.

This is what you should do:
Go to your game installation folder,open RADS folder,open 'projects' folder and there delete 'lol_launcher' folder.After this start your client and it should be fixed.

I hope this helps you guys as it did for me.

or reconnect ur network cable

never tried that it may work the more solutions we find to the problem the better is

Easier solution for possibly everyone, type the "thereisnourflevel" while the undefined launcher's runing.It worked for me and might work for others too.

HAHAH IT WORKS WTF thereisnourflevel!

thanks a lot! safed my day
So just open the window and type "thereisnourflevel" (without " ) and the status will change to online
It still doesn't load after login the "home page" with all the news and stuff, but you can play games