Works great connecting to all my server except for one. The one it doesn't uses a different port because it's connected to the Internet directly. When it does connect all I get is a black screen for about 15 seconds then it disconnects. The standard RDC program from MS works fine to connect to this server. Any ideas?

There could be a couple things going on. Let's tackle a few possibilities.

First, there is a bug in the hosted RDP control that causes the screen to not refresh always initially. I have a workaround that starts a timer and "tickles" the hosted control as if simulating screen resize events which causes it to start working. It's usually kind of random, but it may be that your particular server is more susceptible that most. It's possible the workaround is not working in your case. So we can kind of force the issue by selecting "suspend network traffic when tab is not visible" under the Experience tab. This setting is a network performance setting, but as a side effect, it causes a full screen refresh when the tab goes out and back into view. So, check that, connect to the remote system, wait about 5 seconds, click over to a different tab, then click back and see if the screen comes into view. If so, it's this issue and I can try tweaking my workaround to see if we can cover your edge case.

Second, it may be that you have a dangling session that has frozen up. Sometimes RD Tabs will connect to existing detached sessions and the Windows RDP client will actually create a new session that works. I don't know why, but it happens rarely. Click on Remote Computer Information (the Comp Info button on the Connection page) and see if you have an active session. If so, use the "Log off" button to terminate it. Wait about 10 seconds and try connecting again. Since this system is on the internet, you may not have WMI access to effectively use Comp Info. In that case, log on to the server using standard RDP client, and open Task Manager and see if there are multiple cases of your user. Log them all off. If not, still try logging off the user you logged on as (at least as a test), then try again.

Third and final, if you can try rebooting the remote system. This isn't an effective permanent solution obviously, but I have seen where remote desktop, in general, gets wonky and a reboot clears it up.

Your second paragraph above is what lead me to the solution. In the past, I had noticed but paid no heed to disconnected sessions after logging in using the MS RDP Client. I've been in the habit of disconnecting rather than logging off all my servers. I believe that was/is the problem. After logging on and terminating all disconnected sessions, I was able to successfully log in using RDTabs from my workstation and this has continued to work after repeated attempts. WMI has never been a problem on any of my servers and I use PRTG to monitor important processes on all servers.

Thank you!

Excellent! Glad it worked. Just to be clear, disconnecting sessions is normally never a problem. I do it all the time and only experience this issue maybe once or twice per year, usually on older (2008 R2) servers. But sometimes one will get hung and it has to be logged off to reset.