Locking issue after migrating GAE app from Hibernate 3 to Hibernate 5

We have an existing GAE application working with Hibernate 3 with no issues. After migrating to Hibernate 5, the application started to lock every 6 hours with no logs or stackstraces. The application would work and suddenly requests will start to timeout and GAE will block the application. After we rolled back only the hibernate 5 migration the application worked fine. Is difficult to pinpoint the issue in another environment because is dependant on load and there is no logs that show an error.

Any ideas were the issue could be?