See discussion -
/index.php/topic,10463.0.htmlNotable summary from the entire discussion - It wasn't anything ULX could fix.
Gmod uses 'Awesomium' browser library - It's extremely picky about content it displays.
You must use "
https://site" for https sites. Don't just try non https or no https.
You mention forum working in a custom command.
Try setting same forum link to MOTD URL as a test. If it works, Awesomium doesn't like something in your motd page.
If it doesn't work, well, that's all i know for now.