I have this exact same problem. This has caused our page contact system to break and as a result we have been losing customers for days now, who tried to contact us. We have only just became aware of this.
We are using the latest version of WordPress (5.6) as well as for all the plugins.
Our site is located at https://www.sabusinesscoaches.co.za/ and the problems are found on the pages containing actions and popups, such as https://www.sabusinesscoaches.co.za/services/bcoaching/
I have just installed jQuery Migrate plugin and some previous javascript errors disappeared, but not this one. Help urgently needed.
UPDATE: I have reverted to a previous WordPress version (4.5) and luckily the website is now usable again. However this is of course a temporary fix and the problem will still be there when we upgrade again.
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This reply was modified 4 years, 3 months ago by chrisstefan. Reason: Update with recent actions taken
