I can log into Connect Central, but I can't log into a specific meeting. How can I fix this?

The answer to the second question is the one that stands out. If you are authenticated into Connect and then launch a room and it rejects you, then something is not behaving as it should. 

I'd recommend you have the logs pulled for when your access attempt failed. If the servers aren't hosted with Adobe then you can just work with the group managing the server cluster to get the logs. If it is hosted with Adobe, you will have to work with Adobe Support to get the logs. 

If possible, maybe try removing the account from Connect and re-creating it and see if something about the user account was corrupted and is resolved by refreshing it.

Sorry for starting off with basic questions, but with no history I need to start from the begining.

Is the meeting on the same account as Connect Central? For example, meeting URL is companyA.adobeconnect.com/meeting and Connect Central is companyA.adobeconnect.com.

Are you joining the meeting after logging into Connect Central or do you log out of Connect Central and then join the meeting?

Is your UN/PW being entered automatically by your browser or is it manually entered by you?

We'll go from there