i got that problem too with the little plus that webmin upgrade just
trashed the whole /etc/webmin dir while it encountered it ;p
It seems it was because i use a mix of tagging and real quota on the
same partition but never gone too far into this. It disapeared when i
removed the tagging of the partition ( do not forget to chxid 0 all the
files before remounting without tags).
Regards,
Ghislain.