The response is not a valid JSON response

After trying different settings the plugin suddenly give us this error in the admin using any of the FAQ blocks:

Error loading block: The response is not a valid JSON response.

And on the front end pages where we already added FAQ blocks they now either return nothing where the block is inserted or gives a 500 error.

Examples:

https://beta.elevateapps.io/faq/
https://beta.elevateapps.io/ufaq/are-the-elevate-apps-safe-to-use/

We did not make any other changes to the site than the Ultimate FAQ.

How can we fix this and avoid it happening again?

The page I need help with: [log in to see the link]