I haven't seen this. It would normally mean that either there's no internet connection, GitHub's API is down, or your server is refusing to make the request.
There's a workaround on the docs page
for using the API URL directly and pasting the result into the versionlist file. Search for 2.1.2.
Here are the steps:
As of UGM Version 2.1.2, if you get persistent errors about the versionlist, you can bypass them by creating the file: core/cache/upgrademodx/versionlist and following these steps:
1. Go to this url in your browser: //api.github.com/repos/modxcms/revolution/tags.
2. Copy the entire contents of your browser window (Ctrl-a, Ctrl-c)
3. Paste it into the file (Ctrl-v).
4. Note the name of the latest version at the top of the file (e.g., 2.7.0-pl)
5. Set the ugm_last_check and ugm_file_version settings to the latest version
6. Clear the Site Cache in the Manager
You should be able to do as many upgrades as needed in the next 24 hours