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Go into your System Admin --> Preferences and turn the on the PHP Debug Mode.
Then try to view your topic then copy and paste the error you get. It might be obviopus to you when you read the error, and if not, it may well give the smart guys here and idea of the problem (for which I aint one!!)
Other ideas off the top of my head. Is your templates_c folder write-enabled? If not, it needs to be. Have you got HTTP_REFERRER enabled or not? In other words, try disabling your firewall for a second and see if that works. If it works with the FW disabled then look up HTTP_REFERRER (there are many posts on it) and adjust the code.
The fact that it is happening all of a sudden is likely to be due to one of your SQL tables getting itself in a fluster. A common one to try is the Xoops_session table. Have a look at it using phpMyAdmin, and see what it's status is. If it looks odd then flush it (empty it)
Ted