UPDATE: So dear @Tajrebatee has the following problem:

In the "Embeds providers" field his site had the "Embedly" value. But the "Embed.ly API key" has been left empty. So the call of the embed feature in the Update form was generating the Javascript error due to a non-existing embed object. And this issue was breaking the work of other JS code (yes, the Timeline's "Load more" button too).