The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use .. pool

Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool.
This may have occurred because all pooled connections were in use and max pool size was reached.

My web app written is ASP.NET C# and database use MySQL.

To solve it. Close all your connection each time you have finishing your query.