After digging some more, see the site is behind CloudFlare.   In the last month, I've had to migrate some Moodle sites off one domain, onto another domain *not behind* ClouidFlare.   Site had Google authentication + Google Repsository + Google Document conversions working prior to being placed behind CF DNS.

Behaviors were strange ... could login to the instances, navigate most things *except for anything admin* in it's link.  Would get a CloudFlare 525 error with a diagram drawn by CF indicating nothing wrong with browser end, nothing wrong with CF, but a Big Red X on the server end.    Did all their recommended test from the affected server.   Nothing reported a problem.

I then got down to the nitty gritty ... the NIC ... and in an ifconfig, I see RX packets were being dropped .. that's outbound.   Every test I could do on that NIC passed.   The only thing I did not control ... the configuration of CF CDN and firewall rules.   As I understand CF, they have  both inbound rules and outbound rules.

Something to check with your system?

'spirit of sharing', Ken