An update for anyone who has suffered the same problem. After some research on the internet, I decided to buy a display emulator. Since it was less than €10, it seemed worth a try. I'm pleased to say that this has solved my issue completely - I can now run the system headless but with full graphics output.
I have recently built a new machine which uses a Quadro RTX 4000 graphics card. When I connect a monitor to the machine, everything works fine. When I connect uses RU, everything appears ok initially (graphics all normal), but certain programs do not function properly. For example, when I open MS Word, I cannot then start a new document or open one because the graphics panel is blank. The real issue is when using SolidWorks (one of the reasons I bought the specific graphics card), where again the graphics window is blank. It does not seem to be a software (Word or Solidworks) issue or a hardware issue, because when I swap again to a direct connection to the monitor, all works perfectly. It seems that RU is somehow blocking some aspect of the graphics display. Has anyone else experienced the same type of issues?
Updating to 6.12 Beta has solved the problem with the shift, ctrl an enter keys. However, I now get a lot of connection problems, where the connection screen is blank. The only way I can get a connection again is to connect via a terminal screen, restart the remote computer and then do a new connection. I need to do this multiple times a day and it's very annoying. I have updated the host machine so that both viewer and host are using the same version.
Overnight, my shift and control keys have stopped working when I connect to a remote PC. I see that this is a common problem. I have tried changing to unicode keyboard - this allows me to use the shift key, but not ctrl or enter. No changes have been made to the remote PC (at least, not manually instigated). This makes the software unusable for me which, in the current working from home situation, is a major issue. Since so many people are suffering from this, please make finding a solution a priority.