It seems like one of your existing modules or 3rd party plugins not compatible with Easyblog 5.2.

May i know your current test site (test.themxx.xxx) also having this issue? If no, is it possible for you install this mijosearch extension into your test site and see whether this issue still persists?

If yes, so we can able to test this on your test site.

I have installed a search menu item on the default test site.

It seems like that error coming from MijoSearch extension, you can refer on my attached screenshot below.

I already updated some fix into this file JoomlaFolder/administrator/components/com_mijosearch/extensions/com_easyblog.php , it should work fine now, can you give it a check on your test site.

By the way, can you report this issue to MijoSearch developer regarding this so for the next user who use this will not hitting the same issue.