Cannot access trisquel's forums when using vpn

This looks to me like an issue with bad behavior script that some sites use to prevent spammers. http://bad-behavior.ioerror.us/support/faq/ shows some suggestions.

It has been a long time since I've worked with bad behavior. If I remember correctly, Bad Behavior does not block bad IPs per se, but instead blocks bad configurations, as spammers are not that good at configuring well. (For example, malformed User Agents).

This leads me to believe (unless I'm wrong, of course), that bad behavior does not block the VPN IP but whatever config you have that is wrong, which is what you should check into. What other configurations do you have when you use the VPN than when you are not?

I also believe that badbehavior keeps a log of errors, if any webmaster on trisquel's forums has access to the log and can lookup the issue you're having they might be able to point you in the right direction.

Welcome to the world of preventing spam :/. It's always a balancing act of trying to prevent spammers and not users, clearly it doesn't work perfectly.