Take a look at this forum post https://forum.mendix.com/link/questions/4073. Theres another link that may shed some light on your issue.
Have you added Custom Java Actions? If not, I would suggest deleting the whole app from your machine and downloading it again from the teamserver. This worked for me in the past in that scenario. In that case somehow your app version got corrupted.
If you did create Java actions then they are referring to classes you can normally not access, hence need to be added so the application has at least read rights to those classes. See: https://forum.mendix.com/link/questions/3953