The first step is to check for double JAR files in your userlib with different version numbers. You can delete the oldest version. If that does not solve the issue you can use the Trick of Rolf Bolt in this post: https://forum.mendix.com/link/questions/86721
Another trick would be to set
-verbose:class
as JVM parameter (Project Explorer -> Project -> Settings -> Configuration -> Edit -> Server -> Extra JVM parameters )
This will give you logs about conflicting jar files.
And what I always do myself when doing major Mendix version upgrade is remove all in the userlib and download the modules from the appstore again. If you have no own custom Java in your model a good way to start clean. You still have to do the above because multiple modules have conflicting jar files in the module.
Hope this helps,
Ronald