Did you update mod on server & client? Yep. both running same version exactly travelers-backpack-1.20.1-9.1.9.jar It doesn't happen on every tool switch, but I've noticed that the tools keep getting ...

at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext (ContextLoader.java:383) at org.springframework.web.context.ContextLoader.initWebApplicationContext ...