Folder Covers Slow to Load or Not Loading

I've got ~750 ePubs on an external drive attached to an ubuntu server I'm using. I utilized the LinuxServer.io Docker image to run an instance of Ubooquity. It runs behind another container that acts as a reverse-proxy for a few services I host. I noticed a while back that the covers of the folders weren't loading very quickly. Sometimes folder links would take a full minute to load. I started trying to troubleshoot this today when I had a minute.

First I updated the docker container as I was a version or two behind. No major difference after updating. 

Checked permissions on the Books folder. (specifically that the gid and uid match, and I set permissions to 775) 

Added another theme (plextheme) to see if that would alter anything. 

Changed Maxmem on the container to make sure it wasn't running out of RAM or etc. (4096mb)

Cleared cache on the database

Tried incognito and a new browser just in case. 

Confirmed same results on a mobile browser. 

I use calibre sorting, and it tends to be that all the A-G or so author folders will correctly load an image. I might get one or two folders with a generic cover, and then the rest of the page is showing broken images where the cover should be. 

I checked the logs and am seeing this and others like it: 

ERROR com.ubooquity.d.b - Request processing failed for URI: /books/1969/coverFolder?cover=true (method:GET)
java.lang.NullPointerException: null
at com.ubooquity.provider.book.g.a(SourceFile:169) ~[Ubooquity.jar:2.1.2]
at com.ubooquity.d.c.a(SourceFile:142) ~[Ubooquity.jar:2.1.2]
at com.ubooquity.d.b.handle(SourceFile:54) ~[Ubooquity.jar:2.1.2]
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:190) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1228) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:170) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1130) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:118) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.Server.handle(Server.java:564) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:318) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:251) [jetty-server-9.4.0.v20161208.jar.6468239414284984926.tmp:9.4.0.v20161208]
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:279) [jetty-io-9.4.0.v20161208.jar.1142343236971734481.tmp:9.4.0.v20161208]
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:112) [jetty-io-9.4.0.v20161208.jar.1142343236971734481.tmp:9.4.0.v20161208]
at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:124) [jetty-io-9.4.0.v20161208.jar.1142343236971734481.tmp:9.4.0.v20161208]
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:672) [jetty-util-9.4.0.v20161208.jar.4902311888213625721.tmp:9.4.0.v20161208]
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:590) [jetty-util-9.4.0.v20161208.jar.4902311888213625721.tmp:9.4.0.v20161208]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_242]

Can anyone assist me here? Is it something specific to my docker image? Something I messed up in the settings?