Further investigations revealed actually two independent causes.
1. German umlauts (ä/ö/ü) in the description field lead reproducible to missing letters at
the end of the exported vm.ovf file (sometimes one last character, sometimes the last
two)
A short test (only one) revealed, that the suggested changes to pack_ova.py fixed that
problem.
2. If there are more than 26 .ova files in a directory, the import dialog throw the same
error message, as if there was no file existent at all.