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That precise problem doesn’t ring a bell, but it looks like some of the core file have gotten corrupted.

What I’d do, is first and foremost, to reinstall wordpress.

You can just grab the wordpress.zip offered on ww.wp.xz.cn, and upload those files by FTP to your hosting account’s location.
When asked what to do for files that are already present, choose to Overwrite, Always.
(If needed: https://codex.ww.wp.xz.cn/Installing_WordPress )

That reinstallation operation only overwrites core files, meaning
– you don’t lose your posts
– you don’t lose your media
– actually, the operation is 100% risk-free.

Hopefully, that will do the job for your problem 🙂

Glad for you 🙂

And… show me your fingers now… don’t discuss… *WHACK*
Of course you have to make backups 😀

There are plenty of backup solutions ( https://ww.wp.xz.cn/plugins/search.php?q=backup ), don’t waste a single minute, maybe the next time you have an issue it will compromise your backup or your media, better be safe than sorry, haha.