After upgrading, checkout if any marketplace modules can be updated as well.
And check if the widgets you use can be updated (Menu App > Tools > Update Widgets).
If you have taken does steps and keep the problem, Let me know
In addition to Peter’s answer, Mendix doesn’t clean up your userlib directory.
Make sure that after updating the marketplace modules, you clean up any duplicate/old versions of jars inside the dir: <Your project>\userlib
In case there are issues due to duplicate userlibs, I can recommend https://github.com/cinaq/mendix-userlib-cleaner
read and follow the instructions in the documentation