You need to decrypt your client.
Either download and use UOGateway to connect -
www.uogateway.com
or
Use UORice -
www.cwo333.com/uo_rice.zip
For UOGateway:
Install it
Run it
Select "New Local Server"
You only need to put in a name and an IP/port, nothing else.
Encryption should be "Remove Encryption" by default.
Hit Save Settings.
Now your server name appears. Double click it and try logging in.
For UORice:
Run the program.
It will automatically create a new client using client.exe in your UO Install path. The new client will be in the install path named No_Crypt_Client2D.exe
Use the no_crypt to run UO and log into your server.