By adding or splitting a link the error message 'Invalid format for a length - qokm' appears.
The error message 'Invalid format for a length - qokm' may appear when the spatial framework was adjusted to a geographic system, like WGS84 or GCS_OSGB_1936. Both calculate in degree for ellipsoidal coordinates in Longitude and Latitude and they are both typically limited to Lon=180 and Lat=90. However, the coordinate values exceed (i.e. X=6425743, Y=5808882), because they were allocated by an Cartesian, metric coordinate system and therefore belong to a projected spatial framework. All versions since Visum 12 differ the calculation of link lenghts between geographic and projected spatial frameworks by considering the position of nodes and link polygones on an ellipsoid in case of a geographic spatial framework.
Solution: 1) Open Network -> Network parameter, choose 'Visum (no projections)' and confirm with OK. This is also a workaround when the correct spatial framework is unknown. 2) Open again Network -> Network parameter and choose 'from file'. Now, you can choose the suitable PRJ-file.
Alternatively, the correct PRJ-file can be choosen directly, too. Deactivate the option 'Transform coordinates in case of changes'. Typically, this happens to networks for which the correct metric spatial framework is not apparently known or is inverted with a geographic spatial framework. That's the reason why this error message mostly appears in the English version.