I have forwarded the issue to our development department and asked for their input on the issue. In addition, could you please clarify if there's any firewall/antivirus software installed on the remote Host machines where the issue occurs?
We prepared a test version of an updated Viewer in order to fix the issue: https://www.remoteutilities.com/test/viewer_2019-12-20.zip Please replace the existing Viewer .exe file in the Viewer installation folder (by default it's C:\Program Files\Remote Utilities - Viewer\) to the .exe file above and run it as administrator. Then please try connecting to the remote Host machine, press the left CTRL key for a few times and close the Viewer.
In the Viewer installation folder, the Logs catalog will appear that will contain a log file. Please send it to us to email@example.com.
In addition, when running the Viewer as administrator, see if there is any difference and if the issue still persists. Also, when sending us the log files, please attach the information on the configuration of the machine where the Viewer is running.