Hi There,
Please do the following, please make sure that you did successfully increase your PHP memory limit, because some hosting provider does not allow their users manually increasing the memory limit (depends on your plan), you can use this plugin to check your memory limit, PHP version, and MySQL version. While you’re at it check your PHP version and MySQL version WordPress recommends 5.6 or later.
If that does not work, try switching to a default theme (twenty seventeen), if it fix the issue please reinstall a clean copy of PRO theme.
Also, please do a test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If nothing works, please generate a new set of security keys using the WordPress Security Key Generator. Copy the entire output and paste it in your wp-config.php file replacing the old key.
Let us know how it goes,
Cheers!