This is normally a indication of a connection issue and can be caused by low resources (i.e. not enough free ram to properly esablish a connection) in which case a reboot can resolve as it would free up resources. this however should be a last resort.
Another cause is a poor internet connection to resolve this ask the client to right click his RDP shortcut and select edit. He then needs to click the Expriences tab
and change the internet speed option to satellite.
This will tell the RDP to expect a variable connection and should help its stability as well as better tolerance for delays.
Lastly make sure the VPS is set to never go into sleep mode
Step 1. Right-click on the desktop and then click Personalize.
Step 2. Click 'Lock screen'.
Step 3. Click 'Screen timeout' settings'.
Step 4. Choose Never from the 'When plugged in, PC goes to sleep after'. dropdown menu