The MySQL error max_user_connections (maximum simultaneous user connections) appears when too many simultaneous connections are made to the same database user. The cause is usually that database connections are not closed properly in your scripts.
How to resolve max_user_connections errors
Check every MySQL connection in your website scripts and ensure each connection is closed correctly once the query has finished. Connection pooling, persistent connections that are never released, or scripts that open many connections in a loop are common sources of the problem. Reviewing your code so that each connection has a corresponding close call typically resolves the max_user_connections error.