If you continue to see the error screen when you are playing the webclient, try this...
If You See This, Then Follow These Steps
Step 1: Go to My Computer
Step 2: Click On "C:/" Or If You Have Multiple Hard Drives, The One That Saves All Your Stuff
Then Scroll Down Until You See The File "reddcache"
Step 3: Right Click On The .Zip-ed Cache File And Click Extract Here. When It Starts Up, Make Sure You Pick The "Overwrite" Checkbox
Step 4: Wait Until It's Done Extracting And Make Sure You Remove The .Zip-ed File Out Of The Cache Folder
Step 5: Play And Enjoy
*If you're still having problems, PM me so we can establish a way of fixing it