Network Admin login issue

I have a Multisite with five sites. Up to recently I had no issues but around the end of March I became unable to access my network admin area.

I can login fine to each site, including the primary, but if I try to go to the Network Admin areas I get into a login loop. Example of what happens is below. I just can’t get past this. No error is displayed.

/wp-login.php?redirect_to=https%3A%2F%2Fmy.domain%2Fwp-admin%2Fnetwork%2Fusers.php&reauth=1

I have seen a few threads about this issue but none of them seem to have resolved the problem and those threads have been closed with no resolution posted.

I spoke to my hosts (Siteground) and they told me there was a WP update on the 30th March. They told me nothing on the error logs could provide a clue as to what is happening so they couldn’t help me. I have tried restoring, one-by-one, full backups from before this date. I have also already tried all the usual suggestions of clearing cache/cookies/using different browser etc. None of these have worked.

I have yet to try disabling all plugins and themes, as I don’t really want to impact my live sites until I have tried everything else to try to resolve this first.

If anyone can suggest anything else or has any knowledge of what happened with the previous closed threads I would be very grateful.