SERVER BUSY - ERROR 500

Hi, when I go on the "Subscribe" to subscribe to a new subscription plan gives me the error 500 Server Busy. What's up? Can you tell me what can be the reason?
Not always does, sometimes it works without changing anything in the configuration or Joomla Membership Pro. This is very strange.
Membership Pro version 1.6.9

May depend on the control VIES? I turned off the control and I have no problems

VIES ? You mean EU tax rule ? Talk to me on Skype on tomorrow and we will check it together

Tuan

Yes EU Tax rule.
I had mistakenly connected in global configuration field vat and then was active control VIES. After removing the connection I had no more problems. Not sure that the cause of the problem is related to this but after removing the connection the problem has disappeared. Talk to you tomorrow

OK. Will talk today when you are available

Tuan