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.
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
The url is accessible when you open it in the browser. That's strange.
I don't have issues with the url on our servers. Is it possible that your server configuration can't access https urls? Maybe you have an invalid certificate?
My box can access https. I have other update URL's using https and they connect fine. There were a couple of updates from other extensions this morning that connected ok. The errors are only with compojoom urls'...
I can open the update URL fine too.
Is a bit odd. I have seen this with older versions of Joomla and suddenly they start to connect again.
Don't worry too much about it, I'll just look on your site for time to time.