If you can connect to the network, and browse to a login website it seems like you should be able to login. I would try typing the password in a text editor a few times, sometimes the keyboard can be a little ackward and even being very careful you may be making typos.
As for cache, I don't think I've seen any Opera folder that had temp webpages in it. Opera sucks enough space away as it is. That's a common IE fix, something goes wrong delete everything, and hope for the best I'd more suspicious of cookies before cache. Mess around in the options see if there's any clear or clean options.
What actually happens when it doesn't login? Does it come back with a new website that says there has been a incorrect username and password given?
