Problem/Bug Multiverse is deleting Items?

Hello,
A few days ago I installed Multiverse (Multiverse-Core; Multiverse-Portals; and Multiverse-Inventories) on my Server. I set up a spawn world; Creative World; and the normal Overworld, Nether and End world.
But now whenever someone is leaving the server or just going through a Portal they will lose the Item they are currently holding.
What am I doing wrong and more important what can I change to make this stop?

Thank you for your answers!