First, make sure your Adobe Air client is up to date.
Then try this solution:
- Go to this folder in your PBE directory: LOLPBE\RADS\projects\lol_air_client\releases\0.0.0 .142\
- Delete releasemanifest and S_OK
- Go into the deploy folder in that same directory
- In there, delete the lib, logs, META-INF, and mod folders
- Also delete LolClient.exe, LolClient.swf, and locale.properties
Your LOLPBE folder should look like the attachment below. Note that LOLPBE and the normal server are in different folders. Try right-clicking lol.launcher and running as admin