Upon trying to boot Minecraft, the launcher log gives this error;

"[17:44:08 FATAL]: Fatal error launching game. Report this to http://bugs.mojang.com please!
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source) ~[?:1.8.0_25]
at java.util.ArrayList$Itr.next(Unknown Source) ~[?:1.8.0_25]
at net.minecraft.launcher.updater.MinecraftVersionManager.getInstalledVersions(MinecraftVersionManager.java:187) ~[launcher.jar:1.6.51]
at net.minecraft.launcher.Launcher.cleanupOrphanedVersions(Launcher.java:431) ~[launcher.jar:1.6.51]
at net.minecraft.launcher.Launcher.performCleanups(Launcher.java:495) ~[launcher.jar:1.6.51]
at net.minecraft.launcher.game.MinecraftGameRunner.launchGame(MinecraftGameRunner.java:231) ~[launcher.jar:1.6.51]
at com.mojang.launcher.game.runner.AbstractGameRunner.onDownloadJobFinished(AbstractGameRunner.java:186) [launcher.jar:1.6.51]
at com.mojang.launcher.updater.download.DownloadJob.popAndDownload(DownloadJob.java:122) [launcher.jar:1.6.51]
at com.mojang.launcher.updater.download.DownloadJob.access$000(DownloadJob.java:12) [launcher.jar:1.6.51]
at com.mojang.launcher.updater.download.DownloadJob$1.run(DownloadJob.java:86) [launcher.jar:1.6.51]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_25]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_25]
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [?:1.8.0_25]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [?:1.8.0_25]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_25]"
The game does still boot, but worlds take forever to load.