If you try to submit a known good, real life flight plan and receive errors like:

ERROR: Unknown airway/waypoint UN871

the most likely reason is navigation data incompatibility. Airspace definition data is updated in so called AIRAC cycles, every 4 weeks, named after year and month when they were published (i.e. 0701 was published in January 2007).

Since 2006, new AIRAC cycle data is only published as payware by Navigraph. Part of the virtual aviation community uses AIRAC cycle 0608, which is the most recent freely available (among others from www.avsim.com), and the other part uses most recent AIRAC cycles from Navigraph. Some 80-90% of routes are compatible across cycles, so they work equally well in 0608 and i.e. 0705. The remaining 10-20% of routes cause the problem described above.

vroute.info by default uses the freely available cycle 0608 data. You can change that by going to route page - see below:

Note that in order to use a cycle other than 0608, you will need an access code downloaded from Navigraph. If you previously purchased a particular cycle data set, you can get the code at no extra charge.