REST API access over HTTPS produces 504 status error code

Dear all,
I am calling OpenCalais' Rest API over 
  https://api.opencalais.com/enlighten/rest
and so far, this worked like a charm. However, since a few days, I always get 504 HTTP status results. I played around a bit and observed that these errors do NOT occur as soon as I switch from HTTPS to HTTP: accessing the REST API over
  http://api.opencalais.com/enlighten/rest
works.
Is this a known problem?
Thanks for any hints or feedback!
Kaspar