As reported by Debra Dotson in MDLQA-382:

When I click on the chat link after the initial date/time (even just one minute past) as either a student or a non-editing faculty, I receive the error message "Sorry, but you do not currently have permission to do that ({$a})". I'm thinking it should be possible for students to enter the chat even if they are a little bit late.

If I click on the link for the chat as a student one or two minutes after the chat session began, I get the error message. If I wait 3 or more minutes after the start time, it lets me into the chat, no problem. I've tested this several times with the same results.