OJS 3.2 error: Failed Ajax request or invalid JSON returned


I’m upgrading my OJS 2.4.x to the 3.2.x version, everything went well on the migration process, but, when I try to add a role on any of my journals, the following error shows up:


the same error shows up when I try to add a file on the review process of a new submission.

Is there a solution to this issue?

best regards,

Juan López