qmgr0.dat and qmgr1.dat store the list of jobs that BITS is working on. Any
time a job is updated, created, deleted, etc., one or the other of the files
will be modified. I'm glad that your problem was solved by deleting them,
but it's surprising.
There are a couple of reasons to be cautious about doing this. First,
deleting the job list deletes all of your active jobs, so programs that
depend on BITS will need to detect the situation and create a new job for
their data. Most apps using BITS handle this situation properly, so it's not
a big concern. Second, this method doesn't delete the partially-downloaded
data for your jobs; the data is downloaded to a hidden file in the directory
where the app asked BITS to place the data. So when you delete the file
list, your machine is potentially left with a bunch of hidden temporary files
of the form "BITnnn.TMP". You'll need to clean those out manually.
If the problem happens again, please post the error code that BITS reports
when it fails to start.
This posting is provided "AS IS" with no warranties, and confers no rights.