Q: RDC not connecting since Yosemite upgrade...

I had same issue with Microsoft RDC v 8.0.9.  But the version released 30 Oct (8.0.10) seems to have fixed the problem, though I had to change my connection details to use the IP address of the target Windows7 machine I was connecting to, instead of using the hostname as I had previously.

Presumably you checked the things suggested by the error message?  Is the Windows machine you're trying to connect to your office machine?  If so maybe your IT dept could help out?  Did you previously connect using the Windows machine's IP address or hostname?  If you were using the hostname before and are now using the IP address, make sure you've got the right one - my work Win7 machine appears to have 2 IP addresses but only one works with RDP Client over the VPN connection from home (and it wasn't the first one I tried!). Double-check your VPN connection is working - can you successfully ping the IP address from a command-line prompt on your Mac? 

I am having the same issue.  No changes on my end or work server except my updating Yosemite.

Just to update my own situation, I applied the Yosemite patch (10.10.1) but it still hasn't helped with the RDP issue.

Also, although I got it to work a few times, there are still problems with the RDP connection. But I have come up with a workaround which seems to be reliable.

I can't get RDP to connect to the remote windows machine (Win 7 professional) using the machine hostname, only by using the IP address.  The Win machine I connect to changes IP address regularly.  So what I do is, after making the VPN connection, I open a new terminal window (must be one started a few seconds after establishing the VPN connection) and run "nslookup" to find the IP address of the machine I want to connect to.

Then I have to edit the RDP connection details to use the IP address it's given me (if it's changed since last time I used it).

I can then start RDP and connect successfully.

So it looks to me that there's still a problem with Microsoft RDP 8.0.10 and Yosemite.

NB - if you're using Cisco Anyconnect for the VPN connection, there is a new version which has been marked specifically as "Yosemite compatible".  I've installed this too, though I was still able to use the above technique with an older version of AnyConnect.

Hello,

me too with same issue. Yosemite 10.10.4 installed.

I tried with various versions of Microsoft Remote Desktop, none of them worked!

Thanks

On a similar but slightly different note, I am getting the same error message when trying to connect to my work computer over a VPN via RDP when connected to a secure wifi network (such as the wifi I have at home). Oddly enough, I am able to connect to my work PC when i attempt over unsecured WIFI such as the Xfinity public. My work PC has a static IP set and it also connects flawlessly from any wifi if i use a PC.

In short, the only time i cannot connect on my Macbook is when i am connected to a secure wifi network. Any suggestions would be much appreciated. This is driving me nuts! Thanks

FYI I am using a MacBook Pro with Yosemite 10.10.5 and RDP 8.0.22, both of which are the latest versions.

Was having this same issue after the Yosemite upgrade.  I was able to fix this by going into the Preferences > Security > and selecting "Always connect, even if Authentication fails".