To rule out any theme/plugin specific issue, try:
How-do-I-purge-my-cache-“>such as Cloudflare. That will solve many weird JavaScript issues.
– If that does not resolve the issue, try 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).
– If that does not resolve the issue, try resetting the plugins folder by FTP. Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder (if you have created such folder). The easiest way is to rename that folder to mu-plugins-old.
– If that does not resolve the issue, keep all plugins deactivated and try 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).