My “All Activity” page just broke and it only returns a white page. 

The error log shows multiple new errors all like this:

I see that file (core->modules->admin->system->metatags.php) in backup folders but it is indeed missing from the current installation and a fresh download I tried. 

Thanks, I have fixed this for an upcoming release.

Is there a quick fix?

Not really.

It's also not critical and the file isn't needed at all! It's just a broken link to the manifests.xml "file" in the frontend.

So the fact that my All Activity page fails is unrelated? I also tried a search index rebuild, which fails and now the queue is locked. 😕

There's now a ticket about the other issue because I'll need to take a look at this.