First day's backups worked, now nothing …

I get the “unexpected response: security check” error.

I have the following plugins installed: https://imgur.com/a/NQxhaC5

So. Everything was working swimmingly for almost a week. I was uploading content and suddenly I got an HTTP error 501. OK. I tried different methods of uploading the file through the web interface (GoDaddy WordPress hosting). Got the same error. I went to the plugins to do an update. I can’t do the update, and it errors out. Backups error out. By the way, the GoDaddy folks can FTP in just fine, write, and read files.

I went through the forums in here. I logged out of WP and logged back in. No dice. I disabled caching. No dice. I tried deactivating and uninstalling plugins. It errors out. In short, I can log in. I cannot modify content or plugins. I cannot add content. I cannot delete anything.

I can still go into all of the various options, but no changes are saved. I hope you folks can help! :)