I've been getting this error a lot

and before v2.15 I could try joining again and it would always work on the second try. Since updating to v2.15, if I get this error once it doesn't stop, and I can't get into the server no matter how many times I rejoin.
I own the server, I've tried restarting the server, restarting my client, reinstalling my client, and restarting my PC. The only thing that has worked in the past is to delete my journeymap folder but the issue still persists after a few times disconnecting and joining.

Edit: This is what the server log shows when I try to join;
[23:15:27] [Server thread/INFO]: _Rhythe joined the game
[23:15:28] [Netty Epoll Server IO #2/ERROR]: Could not serialize shetiphian.core.internal.teams.TeamCommand$TeamArgument@12cca971 (class shetiphian.core.internal.teams.TeamCommand$TeamArgument) - will not be sent to client!
[23:15:28] [Netty Epoll Server IO #2/ERROR]: Could not serialize shetiphian.core.internal.teams.TeamCommand$TeamArgument@40a64b53 (class shetiphian.core.internal.teams.TeamCommand$TeamArgument) - will not be sent to client!
[23:15:28] [Netty Epoll Server IO #2/ERROR]: Could not serialize shetiphian.core.internal.teams.TeamCommand$TeamArgument@367f0205 (class shetiphian.core.internal.teams.TeamCommand$TeamArgument) - will not be sent to client!
[23:15:28] [Netty Epoll Server IO #2/ERROR]: Could not serialize shetiphian.core.internal.teams.TeamCommand$TeamArgument@3ada2226 (class shetiphian.core.internal.teams.TeamCommand$TeamArgument) - will not be sent to client!
[23:15:28] [Netty Epoll Server IO #2/ERROR]: Could not serialize shetiphian.core.internal.teams.TeamCommand$TeamArgument@48d41afd (class shetiphian.core.internal.teams.TeamCommand$TeamArgument) - will not be sent to client!
[23:15:28] [Netty Epoll Server IO #2/ERROR]: Could not serialize cpw.mods.inventorysorter.InventorySorterCommand$ContainerClassArgument@6bb5798 (class cpw.mods.inventorysorter.InventorySorterCommand$ContainerClassArgument) - will not be sent to client!
[23:15:28] [Netty Epoll Server IO #2/ERROR]: Could not serialize cpw.mods.inventorysorter.InventorySorterCommand$ContainerClassArgument@32106a1c (class cpw.mods.inventorysorter.InventorySorterCommand$ContainerClassArgument) - will not be sent to client!
[23:15:28] [Server thread/INFO]: _Rhythe lost connection: Disconnected

Edit 2: I found a workaround!! Loading into a single-player world initializes ShetiPhianCore properly and I have no issues joining after loading into single-player first.