I had the same problem. My system clock was f*** for a while (date in 2100) and the db tables contained a lot of crap. I cleaned the table and also emptied the files
/opt/zimbra/logger/db/work/zmgengraphs.state
/opt/zimbra/logger/db/work/zmlogprocess.state
and removed all .rrd files in
/opt/zimbra/logger/db/work/
things seem to be running now
