Synchtube currently uses 3 ports -- 8884, 8885, and 8886.
If you receive the message "Cannot connect to synchtube" it is because you cannot access these ports.
It could be for a variety of reasons:
1. You are behind a firewall, or your router is blocking the ports.
2. If you are trying to access from work or school, your network IT may have blocked the port.
No, but seriously. As far as I know those are the only two reasons. If you can figure out a way to unblock the ports, you should be good.
We are working on a solution to proxy the ports, but it isn't as easy as it seems. We actually tried to push a solution a few times already but... it didn't work.
But yes, we are actively continuing to track this problem and are looking for possible solutions! If you still see this problem despite unblocking the ports, please leave a note here and paste your browser specs too (Firefox 3.6, 4, 5, Chrome, etc.)
EDIT: If anyone is a sysadmin and has any other ideas of why people wouldn't be able to connect, please feel free to help suggest a fix!