Are you sure you're running the correct DW20 version? The recommended version changed not long ago in 1.0.19, while we're currently running 1.0.18. Make sure to change "reccommended" to "1.0.18" and run that version to be able to join us ;)
You can solve your problem, by downloading this fix for spigot! http://ci.md-5.net/job/MCPC-Plus-164/ if you use MCPC+
or this link if you use spigot www.spigotmc.org/threads/arrayindexoutofboundsexception.14826/
:)