When trading with cartographers the server will time out while searching through chunks to get structures for the maps. On my server this occurred when the woodland mansion map trade was unlocked, possibly because a woodland mansion could not be found. The stack was logged on a paper server, but the same bug happens on a vanilla server. The stack trace is as follows:

Server thread dump (Look for plugins here before reporting to Paper!):
[16:50:45 ERROR]: The server is waiting on these chunks: [[( -553,1236) in 'world']]
[16:50:45 ERROR]: ------------------------------
[16:50:45 ERROR]: Current Thread: Server thread
[16:50:45 ERROR]: PID: 33 | Suspended: false | Native: false | State: TIMED_WAITING 

{{Stack:
[16:50:40 ERROR]: sun.misc.Unsafe.park(Native Method)
[16:50:40 ERROR]: java.util.concurrent.locks.LockSupport.parkNanos(Unknown Source)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.IAsyncTaskHandler.bk(IAsyncTaskHandler.java:131)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.IAsyncTaskHandler.awaitTasks(IAsyncTaskHandler.java:120)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:285)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.World.getChunkAt(World.java:330)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.IWorldReader.getChunkAt(IWorldReader.java:87)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.StructureGenerator.getNearestGeneratedFeature(StructureGenerator.java:112)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.ChunkGenerator.findNearestMapFeature(ChunkGenerator.java:69)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.WorldServer.a(WorldServer.java:1661)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.VillagerTrades$k.a(VillagerTrades.java:96)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityVillagerAbstract.a(EntityVillagerAbstract.java:246)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityVillager.eC(EntityVillager.java:808)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityVillager.populateTrades(EntityVillager.java:639)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityVillager.mobTick(EntityVillager.java:154)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityInsentient.doTick(EntityInsentient.java:674)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityLiving.movementTick(EntityLiving.java:2594)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityInsentient.movementTick(EntityInsentient.java:506)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityAgeable.movementTick(EntityAgeable.java:171)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityLiving.tick(EntityLiving.java:2381)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityInsentient.tick(EntityInsentient.java:275)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.EntityVillager.tick(EntityVillager.java:185)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.WorldServer.entityJoinedWorld(WorldServer.java:722)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.WorldServer$$Lambda$3184/1614068161.accept(Unknown Source)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.World.a(World.java:878)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.WorldServer.doTick(WorldServer.java:488)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.MinecraftServer.b(MinecraftServer.java:1239)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.DedicatedServer.b(DedicatedServer.java:417)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:1106)
[16:50:40 ERROR]: net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:933)
[16:50:40 ERROR]: java.lang.Thread.run(Unknown Source)}}