A few people get this error, many have tried to suggest fixes like closing browsers and other programs.
The below I have found is a almost 100% working fix without the need to close any other programs.
I have created a share folder on my dropbox so can add any other programs or tools into this folder. But for this fix follow the below instructions.
1. Download: https://www.dropbox.com/s/k9v7e0c6nfodo9n/LargeAddressAware.exe?dl=0
Open the Program
2. Select an Executable -- Select client.exe in your UO install directory.
3. Check Box the Large Address Aware Flag!
4. Click Save. Restart UOSteam and it will open Ultima Online fine.
Enjoy!
