I am having the same problem, I noticed this today. It was not like this a couple days ago. I keep the system up to date.
I checked the update XML URL and it's there, but something must have changed.
I place this here to verify to CompoJoom support that it is not just you, and hopefully they will look into it.
Here is the error message that I have on the Joomla Extensions Update page.
Warning
Update: Could not open update site #15 "CMC Updates", URL: https://compojoom.com/index.php?option=com_ars&view=update&task=stream&format=xml&id=13&dummy=extension.xml