Hello
Did you have more informations in log files? The error only say that something wrong happens.
- It can be an issue with your configuration ( are you using the sandbox or a real account? )
- The amount of the transaction is not valid ( the amount must be a positive number the form 10.15, maximum of 2 digits after the point )
- Is your <ip_server> visible from Paypal? Or is it an internal IP address ( typically something like 192.168.1.x )
Best regards
Stéphane