second monitor via usb adapter stopped working since upgrade

This is a known issue. They people who make the drivers are working on it. Follow their struggle here.

http://www.displaylink.org/forum/showthread.php?t=1059

There are work arounds to some of the issues you may be facing.

turning on better performance in energy saver, and after logging in, put you system to sleep in >sleep then wake it back up. you'll be able to click and drag again. other bugs pop up after that and if you are experiencing abnormal behavior, first try restarting your computer and second try uninstalling the drivers before posting any help on these forums (since these might be the cause of your issue). but feel free to post whenver (of course).

Hope this Helps!

sorry I editted it with said work arounds.

as my link points out,

Affected machines include:

• 13" MacBook (aluminum late 2008, early, mid, and late 2009)
• 13" MacBook Air (late 2008, mid 2009)
• 13" MacBook Pro (mid 2009)
• 15" MacBook Pro (late 2008, 2.53 GHz mid 2009, mid 2009)
• 17" MacBook Pro (early and mid 2009).
• 20" iMac (early or mid 2009)
• 24" iMac (early 2009)
• 21.5" iMac (late 2009)
• Mac mini (early or late 2009, Server late 2009)
• Mac mini Mac OS X Server (late 2009)