I encountered this problem, from a Windows display client, to an ‘El Capitan’ MacOS 10.11.6 NX server (5.1.44) with a Parallels 11.2.1 Ubuntu 16.04 VM.
When I was accessing the VM window on the MacOS server, from the Windows NXplayer (5.1.44), it would not recognized the meta keys (mod keys) and only type lower case with no control signals. If I worked directly at the MacOS display and keyboard, the meta/mod keys worked correctly.
The work-around (indicated previously): edit (in MacOS)
/Applications/NoMachine.app/Contents/Frameworks/etc/node.cfg
uncomment or create a new line
DisplayServerExtraOptions “-oldeventapi 0”
Then restart the NX service (or reboot)
I’m pretty sure this had been working correctly, earlier in the year, but broke at some point in the last few months.
The release notes say it’s fixed in 5.1.40 https://www.nomachine.com/SU07N00101&sn=TR04L04386
However, that does not seem correct for 5.1.44
It looks to me like this is a problem which keeps coming back.