When you enter the static host route to the linuxconf,
the ifup-routes tries to add this route using

route `linuxconf --hint routing <interface>`,

which expands to

route add -host <addr> gw <addr> netmask 255.255.255.255,

which the "route" command (from the updates/5.2/2.2-kernel)
does not like. The "linuxconf --hint routing eth0" should
print "add -host <addr> gw <addr>" for host static routes
(without the "netmask <netmask>").