Hi @DavidMW,
This isn’t exactly the same problem, but in this post here, it is suggested to rebuild your search index to see if that makes a difference: Search Isn't Working - PHP Warning - #2 by jnugent
Could you give that a try? If that doesn’t work, then it might be best to check your error logs and report here on any relevant errors that might be happening.
Best regards,
Roger
PKP Team