Originally Posted by webairalex

Without knowing the internal working of their system, it appears as though they may have an overly aggressive HTTP firewall which is blocking you due to many concurrent requests, or some arbitrary pattern filter they are using to detect attacks. If you are on a VPS or dedicated server, there should be no reason why they cant whitelist you on the firewall and if they refuse to it may be a sign you need to look for another provider.

Good Luck!