You might want to have a look at this forum post here: Failed Ajax request or invalid JSON returned while adding new user "Add User" - #8 by asmecher - it is for a different version, but the same issue being encountered and the person found out that it was a file permissons issue - so you might want to check your file permissions to see that they are correctly configured. Additionally, if you could provide us with the output from your PHP error log, that may help to further diagnose the issue.
Best regards,
Roger
PKP team