When connecting to a NoMachine server with SSH protocol for the first time, the identity of such server needs to be confirmed for security reasons. However, such confirmation is required for every connection to a NoMachine server from an Android device when the NoMachine application has no write disk permissions on such device.

When this issue occurs, a panel reports the following error:

Can't update the file /sdcard/.ssh/known_hosts.

As workaround turn on disk permissions for the NoMachine application.

Some examples to do that:
Android 12+ Settings > Apps > Special app access > All files access > NoMachine
MIUI 12+ Settings > Privacy protection > Special permissions > All files access > NoMachine

Please note, the NoMachine application requires disk permissions only when the user needs actual access to all files, like for example to upload a file to the server. Such permission can be removed as soon as the fix for this problem (TR04U10843) is released and the NoMachine application will be updated.