According to SFB-NX-7744, no further updates or fixes will be issued for 7.5.3 - 7.5.5 as of Mar 31, 2014
Any release not listed in those two places is no longer maintained and you should upgrade to the latest supported release as soon as possible.
Upgrading to the latest release also makes sure to have the longest support period available.
You can also upgrade to the one before latest release, but then the support period will be shorter.
Development: VB.NET, Tcl/Tk Testing: NX11.0 EAP, NX12.0 EAP
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