All plugins deactivated. What could cause this?

I have an AWS server running four WordPress sites. All plugins have been deactivated on one of the sites and I cannot enable them.

When I try enabling them in bulk, or individually, there is no error and it even says "Plugin activated.", but they are not.

I have tried changing the ACLs temporarily, so that the webserver has full access. I have tried moving directories of plugins that have been recently updated, but nothing works.

I have found that UpdraftPlus backups stopped being produced by this site and a similarly configured one (similar custom-made theme and nearly the same plugins) two days ago.

Also, I have noticed that WP-CLI has not been working properly with these two similar sites over the last couple of days.

I don't know what else to say. I'm completely stumped.

What can I do about this?