The only few times we've seen this from customers so far, it was due to an error in the file they were importing that was most likely created when someone manually edited the old V4 XML files.
For example, here was a recent post:
viewtopic.php?t=16639
If a specific file isn't importing, try opening it in V4, save the process as a new profile, then go to File > Export FFF Profile to save that profile to your hard drive. Open the file in a text editor and check for mistakes in the XML. The most common thing I've seen is <temperatureController name="XXX"> or <extruder name="YYY"> tags where XXX or YYY don't match any other temperature controllers or extruders defined in the rest of the file. Seems like a copy and paste error that someone must have made when manually editing the XML from V4.
In the few cases I've seen, once you remove those extra sections with invalid names, the file can then be improved into V5.