We suggest to start from plugins deactivation but if this won't help, then please re-install theme but thru FTP only to be sure that all files were transferred properly.
I had same issue and for me problem was defering of js, if you have it turned on website level (e.g. in functions.php) try to turn it off. It worked for me.
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it turned on website level (e.g. in functions.php) try to turn it off.
It worked for me.