I have similar issues with resizing textboxes in NagVis on my 2.1.0p5.cre server. Often it gives an error popup, then afterwards it resets to situation before trying to resize, not saving cfg.
Sometimes doing edit map > lock / unlock all helps, but often I just edit the cfg file of the map in ~/etc/nagvis/maps/ manually.
Maybe give OrbVis a go (if you don’t have custom stuff or geomaps) :