It’s hard to say whether the problem is with WordPress site or something not right with Google AdWords system.
I had a client with a simple WordPress website and ads got disapproved saying the destination URL is not working but in reality, it was working fine.
Upon contacting with Google support they made it worse saying something is not right in the HTTP response. When I asked what in particular then they had no idea and it was a waste of time.
After doing some research I found that there was no exact fix as people tried different things like creating a new ad with separating landing page etc.
Following worked for me.
Someone had indicated that if AdWords auto-tagging is enabled and when Google will redirect the traffic and if the system can’t handle query string then it may result in 404 and which may result in ad disapproval.
So, in my situation auto-tagging was not enabled, so the first thing I did was enabled the auto-tagging in Google Analytics.
Then I checked if the WP site could handle the query string like example.com/landing-page/?adwords. And the system could handle it fine.
So, I created few new ads and end of every landing page URL I added the query string like ?adwords. That’s it and it worked.
So, it may or may not work for you depending on your situation, Google is being very vague on this matter.
Some community feedback you can read here: