Sorry I can't offer more info, perhaps the mod isn't built for the breaking changes in forge .45?

IronJetpacks-1.14.4-2.0.0.jar
forge-1.14.4-28.0.45

When this mod is in my client and the server, it blocks being able to connect to a dedicated server-

Server console-

[10:21:20.657] [Server thread/INFO] [minecraft/DedicatedServer]: Goshen joined the game [10:21:20.665] [Netty Epoll Server IO #7/ERROR] [minecraft/NettyPacketEncoder]: java.lang.NullPointerException [10:21:20.682] [Netty Epoll Server IO #7/ERROR] [minecraft/ArgumentTypes]: Could not serialize biomesoplenty.common.command.BiomeArgument@140329d6 (class biomesoplenty.common.command.BiomeArgument) - will not be sent to client! [10:21:20.682] [Netty Epoll Server IO #7/ERROR] [minecraft/ArgumentTypes]: Could not serialize net.minecraftforge.server.command.ModIdArgument@4959edcf (class net.minecraftforge.server.command.ModIdArgument) - will not be sent to client! [10:21:20.684] [Netty Epoll Server IO #7/ERROR] [minecraft/ArgumentTypes]: Could not serialize net.minecraftforge.server.command.EnumArgument@635e608b (class net.minecraftforge.server.command.EnumArgument) - will not be sent to client! [10:21:20.685] [Netty Epoll Server IO #7/ERROR] [minecraft/ArgumentTypes]: Could not serialize net.silentchaos512.gear.api.parts.MaterialGrade$Argument@5730ad72 (class net.silentchaos512.gear.api.parts.MaterialGrade$Argument) - will not be sent to client! [10:21:20.809] [Server thread/INFO] [Treasure Bags/]: Sending 6 bag types to Goshen

Thank you