In mobile version, if there are notifications (there are red numbers on the Alerts icon), clicking the Alerts icon shows "server error". It doesn't happen in desktop version.
It's not app, just the website.
"Mobile version" means, if you visit the forum on a mobile browser, the forum looks friendly on the mobile, though the layout is not same as the "desktop version" (which you will see in a PC browser).
On the contrary, if a website is not mobile friendly (or to say, it doesn't have a mobile version), if you visit that website, the page looks exactly same as the PC version, and it will be very crowded and the font is too small to see. That happens on some old websites.