It seems this is because in 4.3.3, according to the release notes, they added: “Fix — Prevent HTTPS websites from attempting to load plugin assets over HTTP.”
I am having this same JS issue too. Others have “fixed” this by making sure they’re loading their entire WP site over https. The problem for me is, I only load WP admin over https and not my public site –I just need encrypted admin traffic. (To do this, I use the “Force SSL for Dashboard” option in the iThemes Security plugin.)
If you downgrade both The Events Calendar and Events Calendar Pro to 4.3.2, everything works again.
A bigger issue for me is, the JS error also destroyed my recurring events, because when you click “Add Another Rule”, the plugin seems to delete any existing rules and then just saves the event without rules. I just spent an hour recovering my recurring events from a backup.