Seo title Plugin array in title tag

Hi

I am using an mu with multiple sites.

I have Seo Title Tag plugin installed and it is working in all but one of the my sites. The one that isn’t working shows “array” in the custom field on the page or post edit page.
As far as I can see there is no difference between the settings of all my sites
One other plug in (Broken link checker)is not working on this particular site but is working on allthe others.

I am using the same theme on all the sites