Deprecated Registration.php

You need to allocate error. A critical error will be in the servers error log if this log is enabled. If not – see how to do this on your hosting.

Looks like you have enabled debugging in WordPress, this is why you have notifications and something is out of date on your site. And possibly you have some problem with the current version, not all files are from it. Try manual update to the same version:
https://www.ads-software.com/support/article/updating-wordpress/#manual-update
Make sure that you have a backup and know how to restore the site from it quickly if something will go wrong. If you don’t have any backups, probably this is what needs to back tackle firstly.

You need to learn more about debugging:

Debugging in WordPress

Hi @oglekler thank you for your help.

We have followed the steps on the links provided and it didn’t work.

Baing unable to access the wp-admin we were unable to disable the plugins for the site ?? as mentioned in the guide
Manual Update #Manual Update Step 1: Replace WordPress files > 3. Deactivate plugins.

Any suggestion on how to disable the plugins from phpMyAdmin only for a specific site part of a wpmu installation?

You can rename the plugin’s folder, see more here:

FAQ Troubleshooting

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