Connection down?
The availability of an internet connection is checked at startup in Huygens Essential and Huygens Professional to determine if on-line help is enabled.
Moreover the program switches to off-line mode after a couple of failed attempts to connect to the help server. Maybe the server is under maintenance or there are some problems with the connection.
Even if the connection is down you may still read many help articles that are stored in the cache of your computer, but not new ones. That's why some help articles may return an error while others are properly displayed.
If the internet connection was restored during the program execution, you may try to re-enable the on-line capabilities by clicking on the reload icon at the bottom of the help browser. This will try to look on-line again instead of in the cache directly.
In any case, you can always use your regular internet browser (and not the built-in one) to access the help database in the SVI-wiki, going to http://support.svi.nl/wiki