I ran into the same issue recently, but I found an easy workaround. Easy, that is, once I discovered it :-)
Go to the page where the problematic "Edit this System" link appears. Before clicking on it, open up another tab in your browser, and in that tab go to the "My Forum Threads" link under "Community" on the Audiogon home page. When you click on that link you'll probably be asked to log in, even if you had previously logged in. After logging in there go back to the tab having the "Edit this System" link, and it should work.
I would anticipate that issues such as this will not occur after the forum upgrade Mofi mentioned goes into effect in a few weeks. Or if any such issues do occur they would be addressed promptly and efficiently. I say that in part because the software developers who are working on the upgrade have made very favorable impressions on me in a few communications I've exchanged with them recently in connection with the beta version of the new site.
Regards,
-- Al