Version 1.5 update for the Nintendo 3DS or emulators like Citra. Version: 1.5 (Released April 23, 2015).
Fixed an issue that caused disconnects during Random Matchups and Online Competitions . pokemon y update 15 decrypted 3ds eur usa link
Repositories like Myrient (via Erista) host various decrypted 3DS files. Note that you may need to search their "Updates" section specifically for the v1.5 patch. Version 1