"Path field is required" message when path was filled in

I've only seen this problem on one site thus far, so I'm posting this in hopes someone else can replicate it.

On this client's site, about half the time when trying to add a redirect at /admin/config/search/redirect/add, upon clicking Save the form is returned with "1 error has been found: Path" and "Path field is required," and the path field which had been filled in is now blank. Submitting the form again with the exact same data in the path field as before again has about a 50% chance of being accepted or being rejected with the same error. I've just been repeatedly submitting the form until it gets accepted, but this is not an ideal solution for the client.

Any idea as to what may be wrong? Thank you in advance.