To recreate, I set:
mbstring.language = Russian
mbstring.internal_encoding = UTF-8
mbstring.func_overload = 7
I then created an Advertiser with a name using Russian characters, and linked them to a user with Russian characters in the username and 'name' fields
After doing this, in FF3 the top right SwitchUser menu did not work. Resetting the above PHP settings corrected the issue. Clearing out cache in openx/var/cache might be needed for testing.
Also, I tested in Opera but didn't have the issue (perhaps because of browser cache)