Did you do the upgrade on Friday or Monday (Eastern Standard Time)?
If you upgraded on Friday…
The cause was likely the update to Connections which I released on Friday for a few hours.
Part of the issue is the server is very likely running a very old and unsupported version of PHP (for years). I recommend to update your site to run a more recent version of PHP. WordPress recommends version 7.0 due to it being much faster and far more resource efficient. I recommend at least version 5.6 (which is actually going to be unsupported soon if I remember correctly).
I did rollback the update after only a few hours so you must have been one of the few hundred who updated when this issue came to light…
You should be able to delete the
../wp-content/plugins/connections/ folder via FTP. The will let you back in you admin. Now you can reinstall Connections as you normally would. You data is completely safe and will not be deleted by deleting the plugin folder.
I released a revise update on Monday which contains a very minor tweak which should correct the issue for the very old version of PHP. Reinstalling the plugin after you delete it should resolve the issue.
If you upgraded on Monday…
I’ll need an WP admin and FTP login so I can take a closer look. The details can be posted here as a private reply.
My do apologize for the trouble this has caused you!