Wifi Problems?

I've got an unlocked curve 8520. Whenever i'm on a wifi application like twitter or something, everythings working fine. Then about 6-8 hours later when i try to refresh my tweets it says "You are currently on a data plan that does not support this application. If you want to use this application, please connect to a wi-fi network or contact your service provider to upgrade your data plan." The thing is, im connected to a wifi network (trust me it's strong too) so i just end up turning off my phone then turning it back on and everything works normally. untill 7-8 hours later it comes up the same thing. When that message shows up none of my apps work (except default browser. it works perfectly) it's kinda annoying. can anyone help me to understand why, or is it just a side-effect of the UMA unlocking?

Some apps won't work without a Blackberry data plan, if you have a Blackberry data plan then contact your carrier and ask them why it's not working like it should and if not then i suggest you to get a Blackberry data plan...
You also said that the device is unlocked then it can be also related to the service books, make sure that the service books on your Blackberry are those that used by your carrier, you can also contact them and ask them to send it to you.