document screen lock issue with NX

rhel8/index.md

@@ -27,10 +27,6 @@ It looks like it works already a lot out of the box, but no guarantee can be giv
 
 ## Caveats
 
-### Workstation Installation Takes Long and Seams to Hang
-On the very first puppet run the command to install the GUI packages takes up to 10 minutes and it looks like it
-is hanging. Usually it is after the installation of `/etc/sssd/sssd.conf`. Just give it a bit time.
-
 ### Missing or Replaced Packages
 
 [List of packages removed in RHEL 8](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html-single/considerations_in_adopting_rhel_8/index#removed-packages_changes-to-packages)
@@ -44,6 +40,20 @@ is hanging. Usually it is after the installation of `/etc/sssd/sssd.conf`. Just
 | `devtoolset*` | `gcc-toolset*` | |
 | `git-cvs` | - | `cvs` itself is not supported by RHEL8, but available through EPEL. Still missing is the support for `git cvsimport`. |
 
+### No Gnome Screen Lock with NX Server
+On machines running NX Server Gnome does not provide a screen lock option.
+Still the screen can be locked running below command in the terminal:
+
+```
+dm-tool lock
+```
+
+The underlying problem is
+- `gdm` is the standard Display Manager providing the login screen and the screen locking
+- NX server fails to start Gnome with `gdm` and suggests to use `lightdm` instead ([knowledge base entry](https://kb.nomachine.com/TR08Q09375))
+- so NX server machines now use `lightdm` as Display Manager
+- Gnome does not integrate with `lightdm` screen locking
+
 ### Installing a Machine with YFS and an Old Package Snapshot Tag
 After the `pli-firstboot` script it stopps due to errors (installed yfs kernel module version does not fit running kernel). Still it manages to install everything fine.  
 So please restart the machine (so it loads the right kernel) and run `puppet agent` again to finally start and enable the `yfs-service`.
@@ -57,6 +67,10 @@ This needs to be fixed manually:
 - run the complaining command and approve (or use `--yes`)
 - run `puppet agent -t` to finalize the configuration
 
+### Workstation Installation Takes Long and Seams to Hang
+On the very first puppet run the command to install the GUI packages takes up to 10 minutes and it looks like it
+is hanging. Usually it is after the installation of `/etc/sssd/sssd.conf`. Just give it a bit time.
+
 ### "yum/dnf search" Gives Permission Denied as Normal User
 It works fine beside the below error message:
 ```