Thanks for updating the thread.
Okay, there seems to be some confusion over here. The latest version of X theme is 6.0.4. As you have mentioned that the problem started to arise upon upgrading to WordPress version 4.9.6. To see if I can replicate the issue on my local setup I upgraded WordPress to version 4.9.6 (was running previous version of WordPress ) and then revoked the licence from X > Validation > Revoke Validation and site got validated.
Do you see any error message while validating? I suggest you to please take a look at following article for the common error message and suggested solution for the same.
Please let us know how it goes.