Pro sites: Paypal express button returns 503 error

Hi,

I'm trying to get pro sites working. Everything looks fine, till I press the Paynow with Paypal button. I get the following error after 30 seconds:

'Error 503 Service Unavailable
Service Unavailable

Guru Meditation:
XID: 2023727279

Varnish cache server'

No clue, google didn't return me much on this error. I get the error with my sandbox and live account.

Any help really appreciated! Thanks in advance,
Pieter