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.