Original message details are unavailable. This might be more related to your VPN than anything.
I very frequently use a VPN and have experienced no such issues.
Agreed. I'm willing to bet this is either a connectivity issue or some issue with your VPN itself, as there's nothing in place on the RSOF to interfere with VPNs - Jagex understands that some players may need to use them, according to Mod Tyran in a Reddit thread:
I can post on the forums using Hotspot Shield. I don't know if there is a known issue with other VPNs blocking the connection. There might also be something else on your computer like a web browser plugin or your antivirus blocking the website. Hopefully you don't have a virus infection that's causing this to happen.
Try flushing your local dns and clear your web browser cookies and cache files. If that doesn't work, check if you can post from a different web browser.