If you're using UOAssist & 2D, when you start UOAssist, go into its "Settings" first, instead of hitting "Continue", and change the method of loading.
I had this issue with the recent update of one of my net providers to a Kaspersky-based security suite.
If you don't use UOA, I don't have any idea on your issue, other than for you to check your firewall software, and your router's firewall, to make sure you're letting all the UO files through (typically, anything with an .exe extension in the folder where your UO software resides)