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Have you tried:
– Flushing any caching plugins you might be running, as well as server and/or browser caches. Not just your browser, but any op cache or content network cache as well such as Cloudflare. That will solve many weird JavaScript issues.
– deactivating ALL (yes all) plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
– resetting the plugins folder by FTP.
– switching to the unedited default Theme (Twenty Sixteen) for a moment using the WP dashboard to rule out any theme-specific issue (theme functions can interfere like plugins).