Uhmmm, not sure then. Is everything working well? Maybe there is a configuration issue… have not seen that before personally although there are several posts in the CoP with the same issue. Maybe you can check those and see if that fixes it.
In addition to @jaime.bosque’s recommendation, I would say to please check the access rights for the optionsets.
If that’s the Import/Export app then please choose the Import report mode to be debug and see if you get more details about the error. It’d also help to share the Catalina.out log (without sensitive info) and see if there are related errors which explain why there is an ‘internal server error’.