A non-numeric value encountered

One particular page on my website has suddenly started giving me the following error whenever I try to open the block editor
An unexpected error occurred.
A non-numeric value encountered

and the top bar loses all items (just a blank page with the message and a blank bar at the top with the Concrete5 logo).

It only occurs when I try to edit the actual page contents, so not when I open the Page Settings menu. The browser console (Firefox) says
"TypeError: b(...).tooltip is not a function."
e https://mysite.com/updates/concrete5-8.4.3_remote_updater/concrete/js/app.js:5
    start https://mysite.com/updates/concrete5-8.4.3_remote_updater/concrete/js/app.js:5
    <anonymous> https://mysite.com/index.php?cID=195:48
    jQuery 4

and Chrome says
Uncaught TypeError: b(...).tooltip is not a function
    at e (app.js:5)
    at Object.start (app.js:5)
    at HTMLDocument.<anonymous> (messages:48)
    at i (jquery.js:2)
    at Object.fireWith [as resolveWith] (jquery.js:2)
    at Function.ready (jquery.js:2)
    at HTMLDocument.K (jquery.js:2)
Failed to load resource: the server responded with a status of 500 () messages:1

(messages is the page name).

I have tried rolling back to the previous version, deleting the latest version, and clearing my site cache and I still get the same error. The error occurs in Chrome as well. I have not applied any updates or made any other CMS-wide changes since last editing this page.

Any help with this would be appreciated.

My environment info is
# concrete5 Version
Core Version - 8.4.3
Version Installed - 8.4.3
Database Version - 20180716000000
# concrete5 Packages
Add and Remove Classes and IDs (0.9.5), Automatic Email Obfuscator (2.0.1), Breadcrumb Trail Page Links (1.1), Customize Editing Interface (1.0.1), Editor Comment (7.0.1), Honest Websites Back To Top (1.1.0), HTML5 Audio Player Basic (2.0.7), HTML5 Audio Player Pro (2.1.2), List files from set (1.0.13), Login Page Background (0.9.1), Open Graph Tags Lite (2.1.3), Page Activity (0.9.3), Quick Tabs (1.0), Responsive Theme (2.1.9), Thumb Gallery (1.0.3)
# concrete5 Overrides
# concrete5 Cache Settings
Block Cache - On
Overrides Cache - On
Full Page Caching - On - If blocks on the particular page allow it.
Full Page Cache Lifetime - Every 6 hours (default setting).
# Server Software

View Replies: View Best Answer
Gondwana replied on at Permalink Best Answer Reply
I'm wondering if your host secretly upgraded the php version. They do that sometimes.
MRPockets replied on at Permalink Reply
That was exactly the problem. I would never have thought to check that, thank you!
ConcreteOwl replied on at Permalink Reply
Sorry Gondwana, we cross posted...
Gondwana replied on at Permalink Reply
@ConcreteOwl: if we thought the same thing, I'm proud of myself! I'm loving your work here recently.