After I moved some OpenVZ-containers to a new server, the output of "df -h" and "du -sch /" was more different than I ever expected. I guess it was because of some old vzqota-files, because I used earlier existing VEID's.
The solution: Let the quotas get recalculated with following one-liner. Sorryly you've really to switch the OpenVZ-container for a short time offline:
# vzctl stop 199; vzquota drop 199; vzctl start 199
Replace the number "199" with the problematic VEID - now used&free diskspace should be displayed again correctly.