Lyx 2.0 can't read Lyx 1.6.x documents

[Ralf Glaser, track IT]

I'm running Lyx on a Windows XP system and tried to upgrade from 1.6.7 
to 2.0.
Unfortunately Lyx failes to open 1.6 files with the following message:
“\Buffer_convertLyxFormat.hXXXX is not a readable LyX document”
I tried upgrading to 1.6.9 before moving on to 2.0 with the following 
result:
1.6.9 can handle 1.6.7 files.
2.0 can't handle files created with any 1.6 version.

Any suggestions welcome as i'm really stuck here...