The error message is a bit misleading.
This is most likely related to the certificate used by the browser to make the the HTTPS connection from the PDF to the Rights Management server.
You have to make sure the certificate is installed properly in the browser you're using to avoid this error message.
One way to quickly test is to hit https://<rights_management_servername>:<port> and see if the browser gives you a prompt to install a certificate. If it does, then install it.
Close the browser and try to hit that URL again. You want to make sure you can hit https://<rights_management_servername>:<port> without a prompt to install a certificate.
This is actually what Acrobat/Reader is trying to do. Acrobat uses a browser object and if it can't connect to the the URL without a prompt, it'll throw the error message you're getting.
I hpoe this helps.
Jasmin, thank you for your answer. I think I do not know the proper port number, because the policy server either returns a blank page in my browser, or does not respond (time out). I tried it with port numbers 443, 9443 and 1234 (learned from googling this info). And tried it with both Safari and Firefox.
What I do not understand is why the PDF's are usable without adjusting anything under Windows and Linux. Is there some kind of certificate that these platforms have by default, and that is missing from Mac OSX?