@junkhar, you may take a look at this article How to solve black screen and other possible problems on headless machines
@amigabill, in order to further investigate the issue can you please collect and send us server-side logs? You can follow the instructions explained in this article: https://knowledgebase.nomachine.com/DT11R00181#1.1. Please submit them to forum[at]nomachine[dot]com adding the title of the topic as the email subject. Thanks!