I have the same problem and it seems to be related to the H.264 codec. Activating all “disable” options (or a subset of them) in the client display settings does not help. After enforcing the VP8 codec by setting
EnableDisplayServerVideoCodec 1
DisplayServerVideoCodec vp8
in /opt/NX/etc/node.cfg the problem is gone. I am using a resolution of 3200×1800.