> 
> By the way: did you try --continue to continue the
> mirror ? Another hint is to use -qiC1 when updating,
> after erasing doit.log (--continue will attempt to
> get previous settings from this file - if you
> specify the site's address and options again, you
> have to remove this file)
> 

switched to separate cache directories for each webserver and now using
'--continue'. Everything's working now.
Thanks Xavier for the hints - and for a fine peace of software :-)