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Never mind, got it working with --remoteadmin an --adminport, thanks!

Port 2202 is running fine, port 2203 is giving me the "connection refused" error. Is this normal? How do I add/manage stuff now?

This works! Thank you so much!

sudo java -jar Ubooquity.jar --headless
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Running with the following parameters - headless: true, libraryport: -1, adminport: -1, remoteadmin: false, host: null, debug: false, execdir: /opt/ubooquity
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - #############################
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - # Starting Ubooquity #
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - #############################
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Java version: 1.8.0_212
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Java vendor: Oracle Corporation
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - java vm name: OpenJDK Client VM
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - java runtime name: OpenJDK Runtime Environment
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Max heap size available: 247 MB
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - OS name: Linux
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - OS version: 5.4.51-v7l+
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - OS architecture: arm
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Loading local version info
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Ubooquity 2.1.2 built on 2018-10-14 at 12:58
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Creating application folders
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Loading preferences
20200917 10:20:33 [main] INFO com.ubooquity.Ubooquity - Saving preferences
20200917 10:20:34 [main] INFO org.eclipse.jetty.util.log - Logging initialized @2974ms to org.eclipse.jetty.util.log.Slf4jLog
20200917 10:20:34 [main] INFO com.ubooquity.b - Starting internal server...
20200917 10:20:34 [main] INFO com.ubooquity.b - Reverse proxy prefix is not activated
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Scanning books directories
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Finished scanning books directories, populating folders content cache
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Books folder content cache populated
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Scanning comics directories
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Finished scanning comics directories, populating folders content cache
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Comics folder content cache populated
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Comics statistics: [total: 0, added or updated: 0, removed: 0]
20200917 10:20:34 [Scanner thread] INFO com.ubooquity.data.feeder.a - Books statistics: [total: 0, added or updated: 0, removed: 0]
20200917 10:20:34 [main] INFO org.eclipse.jetty.server.Server - jetty-9.4.0.v20161208
20200917 10:20:34 [main] INFO o.e.j.server.handler.ContextHandler - Started o.e.j.s.h.ContextHandler@1695b06{/,null,AVAILABLE,@admin}
20200917 10:20:34 [main] INFO o.e.j.server.handler.ContextHandler - Started o.e.j.s.h.ContextHandler@11bf319{/,null,AVAILABLE,@library}
20200917 10:20:34 [main] INFO o.e.jetty.server.AbstractConnector - Started library@1bc27f5{HTTP/1.1,[http/1.1]}{0.0.0.0:2202}
20200917 10:20:34 [main] INFO o.e.jetty.server.AbstractConnector - Started admin@1b759d6{HTTP/1.1,[http/1.1]}{127.0.0.1:2203}
20200917 10:20:34 [main] INFO org.eclipse.jetty.server.Server - Started @3413ms
20200917 10:20:34 [main] INFO com.ubooquity.b - Internal server started. Listening on port 2202 (library) and 2203 (admin)
Tyep Q then ENTER to exit
20200917 10:20:35 [Timer-0] INFO com.ubooquity.Ubooquity - Checking latest available Ubooquity version from website
20200917 10:20:35 [Timer-0] INFO com.ubooquity.Ubooquity - Latest version available: 2.1.2 - Current version: 2.1.2