Do you get this error for all actions? or only when trying to save menu font? This is more likely to be a server issue. You could have some corrupted files or some table in the database needs optimization. Possible solution are:
-Download a fresh copy of wordpress files and replace them (look up tutorials to make it the proper way).
-Redux framework requires PHP version > 5.5. Make sure you have the right PHP Version installed on your server. Ask your host providers about it.
– Go to your wordpress installation and make a database repair operation. Find all (and more) information here.
Note: Always make sure to have a backup of everythig!
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