mobs not respawning after some time

Hi all,

I have a problem. When I restart my server, everything works fine. Mobs are spawning naturally (I have disabled mob spawning through spawners with a plugin) in darkrooms. But after some time they stop spawning (no zombie spawn in darkroom anymore).

Also when I enter the command /killall zombie I get an error:
"Cannot measure distance between World and World_Nether"

But few minutes later the command above works again ...

there is nothing in the logs.

Any idea?

PS. I have lots of plugins but didn't update anything in the last time and this issue happend as of sudden.