I had some trouble with my LAN, and for that reason xvnc was failing to launch for some time.
Now it seems like it has blacklisted a lot of valid DISPLAY_NUMBER values, so I'm seeing the following:

/bin/vncserver :263 -geometry 800x600
vncserver: Illegal display number (263). Value must be 1..99.

/bin/vncserver :264 -geometry 800x600
vncserver: Illegal display number (264). Value must be 1..99.

The plugin recommends me to enable the 'cleanup before start' option, but it is already enabled.

1) There should be a setting for an upper limit to DISPLAY_NUMBER (or the limit should just be hard-coded to 99)
2) There should be a way to view/edit or at least reset the blacklist