Leveler has now been removed from 2.1.3-alpha. It is a mistake that it is still available in the Effect Menu, but that problem is logged at http://bugzilla.audacityteam.org/show_bug.cgi?id=1453.
I don’t have a problem running the Nyquist Comb filter or Nyquist Pseudostereo in Oct 4th 2016 2.1.3-alpha after doing so in 2.1.2 release or 15 Jun 2016 2.1.3-alpha. If you do, I think the problem will only occur when you first launch the effect.
If the problem continues, exit audacity and delete the file Users<your username>\AppData\Roaming\Audacity\pluginsettings.cfg.
If the problem continues after that, please give us the exact file name of the plugins and tell us where you obtained them from, and tell us what language Windows is running in and what language Audacity is running in (look in Interface Preferences).
Gale