WordPress: How to fix "Unexpected token in JSON" error when saving or updating a post?

Question

JSON Web Token

Hi friends!
A month ago, after updating WordPress, when loading the main image and saving a new (or updating an old) post, an error began to appear: Updating failed. Unexpected token in JSON at position 7876 (here the last digits are different every time) . I didn’t find the cause of this error and I didn’t find an intelligible answer on the Internet, how to fix it, either :(

I would be grateful for your advice!
Thank you!