wordpress connection error in rds from ec2

Question

They are two EC2 virtual machines, which use wordpress, both connect to an RDS Database, which one of the virtual machines began to mark a connection error.
Both have the same security group, the connection ports were verified.
The machine that has the error can connect to other databases.
The error only occurs when connecting to RDS.

Answer

What is the exact error code that you are getting?  There are different possible scenarios, have you checked: 
- Are both EC2 on same subnet? If no, check routing.
- If software version differs? (php, mysql client)
- Have you compared wordpress configuration files?
- Is the connection user the same for both ec2 and mysql user permission is allowing connection from both/any ip? 
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wordpress connection error in rds from ec2

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