server crash - null pointer exception

Server is crashing at startup with:

// Hey, that tickles! Hehehe!

Time: 11/5/15 7:30 AM
Description: Ticking entity

java.lang.ClassCastException: ahw cannot be cast to ahn
at aqo.a(SourceFile:148)

...

– Entity being ticked –
Details:
Entity Type: Zombie (zf)
Entity ID: 263
Entity Name: Zombie
Entity's Exact location: 491.51, 73.00, 394.49
Entity's Block location: World: (491,73,394), Chunk: (at 11,4,10 in 30,24; contains blocks 480,0,384 to 495,255,399), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Entity's Momentum: 0.00, -0.08, 0.00
Entity's Passengers: []
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at ahn.k(SourceFile:1349)
at lq.k(SourceFile:552)

Attaching some complete crash logs.