"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"

"Connection Pooling allows an application to reuse open connection handles, which saves round-trips to the server."
This occurs when you try using more than Max Pool Size connections. By default, the max pool size is 100. If we try to obtain connection more than max pool size, then ADO.NET waits for Connection Timeout for the connection from the pool. If even after that connection is not available, we get the above exception.
Solution(s):
1. Very first step that we should do is – Ensure that every connection that is opened, is Closed explicitly. At times what happens is, we open the connection, performs the desired database operation, but we do not close the connection explicitly. Internally it cannot be used as available valid connection from pool. The application would have to wait for GC to claim it, until then it is not marked as available from pool. In such case, even though you are not using max pool size number of connection simultaneously, you may get this error. This is the most probable cause of this issue.
2. Increase Max Pool Size value to a sufficient Max value. You can do so by including "Max Pool Size = N;" in the connection string, where N is the new Max Pool size.
3. Set the Pooling Off. Well, this indeed is not a good idea as Connection Pooling puts a positive performance effect but it definitely is better that getting any such exceptions.