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Hello I'm a newbie to this forum, I'll try my best to describe my problem but if anything is missing please kindly let me know and I'll add it!
Recently I've been setting up a few virtual machines using VirtualBox on my Slackware 14.1 machine
I use VirtualBox 5.1.4 r110228 and has the extension pack installed
there are 5 windows guests (all Window 7 Pro 64bit) and 1 slackware guest (14.2) that are currently running on the host, all of which headless
We use all of the guest machines via RDP from Windows machines, some are Win 7 and others XP
As I was testing RDP, I noticed that I can connect to all of the Windows guests via machines running Windows 7, but none if connecting via our two XP machines
I'd get an error message immediately as soon as I click connect (without trying to connect) which says,

Remote Desktop Disconnected
The client could not establish a connection to the remote computer
The most likely causes for this error are:
1) Remote connections might not be enabled at the remote computer --> which is crossed off since other win7 machines can connect fine
2) The maximum number of connections was exceeded at the remote computer --> which is crossed off too there isn't any other connections
3) A network error occurred while establishing the connection --> but win7 machines connect alright...

I'm sure I entered the correct username and password and left the 'Domain' field blank
All of the above talked-about machines physical or virtual are all on the same network

I tried searching on Google but didn't have much luck in understanding what might've gone wrong
Please would anybody give me some hints? Thank you so much!

Jessie wrote:3) A network error occurred while establishing the connection --> but win7 machines connect alright...

But this is not a Win7 machine, is it? I'm not sure but does XP support the necessary RDP version for this? Maybe it is too old to support something this new?

Thank you for contributing ideas Socratis!
Hmm but the same XP machine can RDP to a Win10 (real not virtual) machine though
I could be wrong, but I would've thought that the computer initiating the connection would see its target vm as a real machine?

Not from what I know. Wait a minute, are you using VirtualBox's RDP or the guest RDP?

The guest RDP I believe..

Guest RDP is accessed via the guest's network name or IP address. Virtualbox's RDP is accessed via the host's IP address and a port. Which one are you using?

We have a Windows 7 Virtualbox host running various guests. Right after I updated my workstation to Windows 10 I found that I could no longer RDP into the guests on the W7 host via guest-native or Virtualbox RDP. Turns out some updates to the W7 host for RDP were pending. Once I ran those updates on the W7 host I could remote into the guests with my Windows 10.

Is your Slackware host completely compatible with the new MSRDP protocol? Have your Windows guests been updated?

Guest RDP and accessing via the guest's IP address (i.e. the exact same method & values were used on Win7/XP machines on the network to RDP to the Win7 VM Guests)
Since Win7 to Win7 VM doesn/t have any problem I would've thought my Slackware host is compatible with the MSRDP protocol, but perhaps I should check any RDP-related updates for the XP machines indeed!
Lemme give that a try and get back here

Turns out all I had to do was to configure my RDP access properly on the guest vms...
I selected --> "Allow connections only from computers running Remote Desktop with Network Level Authentication (more secure)" in place of the "..(less secure)" option
Now having selected the less secure option, RDP from XP machines work

In the mean time I also did a wee research in case the 'less secure' option is not an option -

It is possible to enable Network Level Authentication for Remote Desktop on XP machines ( https://soeithelp.stanford.edu/hc/en-us ... indows-XP-)

Or download XP SP3 or RDP 6.1 Client - Network Level Authentication was introduced in RDP 6.0 and supported initially in Windows Vista. It uses the new Security Support Provider, CredSSP, which is available through SSPI in Windows Vista. With Windows XP Service Pack 3, CredSSP was introduced on that platform and the included RDP 6.1 Client supports NLA; however CredSSP must be enabled in the registry first. ( http://support.microsoft.com/kb/951608)

Thanks all who have contributed!!

Hey, thanks for reporting back! Nice detective work there!