I am using UltraVNC Server and UltraVNC Viewer for years without problems, but now I am really desperate after investigating and trying many, many hours.
System:
Windows 10 at the remote ("Server") site and also on Windows PC at home (but behavior is the same using Viewer with Win 11 or from Android cellphone).
Version of UltraVNC is the latest and fresh installed (with default values) on both Server and Viewer side.
Problems:
For instance at starting point I could connect the UltraVNC Server and Screen of Remote PC comes up as it should (but it is new, that I often have to connect 3-4 times that screen does not be black, despite the UltraVNC Server Icon goes to "yellow", showing that I am connected. This is the same, when connecting with TightVNC Viewer).
The bigger problem is, that e.g. 2 hours later I can not login any more. "Authentication failed" - the password is written 100% correct. It does not work, until I change the password on the UltraVNC Server (using "normal" password, no MS-Logon or so).
But the "old" password was new! I did this 20 times, always changing the password as only solution.
From time to time I even can't connect the VNC Server, I have to shut it down (using Helpwire) and start it again (with administrator privileges). A new start of the Server PC also does the job.
So the system has become unusable for me... there was no change on any software applications, maybe a Windows Update, no more (Remote PC is used "only" for house control, but extensive).
I am not a computer expert, but did have a look deeper in the configurations - as mentioned I have only default installations of both UltraVNC Server and Viewer and it worked for many years (but same behavior with TightVNC Viewer for connecting).
The first time I looked at the server side on the "log" panel. There I can see, that permanently different foreign IP's (bad) are trying to connect to the VNC Server. Started to give this IP's into the rules with "refuse", but the new IP's trying are faster than I could write it into the rules (refuse for IP works of course).
I never looked at that log, but I think that attacks have been always there. The password is strong, and after many tries the IP is going on a blacklist, as far as I understood.
But I must open Port 5900 on the Router/Firewall - otherwise myself could also not connect from outside
Windows Defender Firewall is active (as default installation).
The problem is, I can't figure out a real system. Sometimes I can login after 1 hour and password failed, sometimes I can login the next day, than again after a few yours password becomes invalid again.
So I really have no idea, how to get it working stable. As mentioned, the system works for years and problems come up suddenly one day.
Thank you so much for reading.
Best regards
Gun