The PBE has been rolled back to patch 3.7, the patch currently on live, in an effort to provide a quicker fix for the problems currently plaguing the live servers.
Here is vLemon with details:
Many of you might know that some of our live environments are experiencing stability issues. We decided to change the version that we had on PBE, to the one that we have in live in order to test fixes for those problems. That means all the content of the 3.8 patch (Aatrox + skins) will be removed.
If you have any problems after today's patch (28th of May) please let us know!
We really appreciate your help and we apologize for the inconvenience."