I have spam in my console when players joins, I know, its problem of multiverse, but it is possible to change way of saving itemflags?
I dont know about any problems with it. I have just spam in my console.
This happens only when i have itemflags on items.

Thanks for your awesome support.

ItemJoin 4.6
Multiverse-Inventories version 1.4

CraftBukkit version git-Spigot-7d78b81-d5e7885 (MC: 1.11.2) (Implementing API version 1.11.2-R0.1-SNAPSHOT)

3.) What is the problem that you are having?
Errors: https://pastebin.com/Fj0MigiD
Config: https://pastebin.com/x2uDKfSe

This error has nothing to do with ItemJoin, if a player gets certain items in their inventory it will throw those errors regardless of ItemJoin being installed or not.

Multiverse-Inventories as of MC 1.10 has become unsupported. I would suggest moving to PerWorldInventory. https://www.spigotmc.org/resources/per-world-inventory.4482/

Wish I could say otherwise but the dev doesn't seem to be active anymore.

Ok, thanks.

Sorry, also there is no way of me changing how itemflags are handled because its all done through bukkit/spigot's API.

Ok, no problem :)