Can't log in to forum

I'm at the office. I can log in to this forum from that machine. The machine I normally use, my MacBook laptop, I can't.

When I can't log in -- I direct my browser to MuseScore.org, click on the "forums" link, click on the "Log in to post a new message" link. I get sent to the portion of the site where I have uploaded some scores. I am logged in there, but that's not where I want to go. I navigate back to the forum, where I am not logged in.

The two computers use different versions of Safari for a browser. One runs Mac OS 10.7.x, the other is on Mac OS Yosemite. I can log in with the OS 10.7 machine, not the Yosemite machine. I'm not experiencing this problem on any other websites, and both machines appear to be working normally in every other respect.

Please advise.