This is quite a huge leap. There are many, many problems that could be caused by things you forgot to change in your own code/build. Also, Hibernate Search 5.6 is quite old and not actively maintained.

I'll close this ticket. Please open a question on our discourse forums or on stackoverflow with if you want some help. Make sure to include the version of all your dependencies.

Some advice:

• Migrate by steps. In your case I would advise, at the very least, migrating to Search 4.0 first, then 5.0, then 5.6. Have a look at the migration guides: https://hibernate.org/search/documentation/migrate/
• Make sure to use compatible versions of your dependencies. You can find the compatible dependencies for Hibernate Search 5.6 here: https://hibernate.org/search/releases/5.6/#compatibility