Webmin reads /etc/route.conf and builds a HTML Page with routing entries.
When a new route is entered and saved it is written immediately to /etc/route.conf.
Nothing wrong so far.
When issueing a '/sbin/init.d/route restart' (which consists of 'route stop' and 'route start').
'route stop' does the following, which produces an error: It reads all entries in /etc/route.conf and removes them from the kernel routing table in reverse order. Because Webmin appended a (or some) new entry(s) to /etc/route.conf , /sbin/init.d/route tries to delete the newly Webmin generated route too. The Problem here is that this particular route wasn't activated in the kernels routing tables yet so /sbin/init.d/route gives an error and exits before doing a 'start' again.
Cheap Workaround is to reboot the machine.
It would be better to make Webmin to activate the new route(s) in the kernel routing table too when /etc/route.conf is modified by Webmin. This should make a '/sbin/init.d/route restart' possible which saves you from reboot (Which you should only do to add/remove new hardware or use a modified kernel).