Hello
Incremental conversions require both VSS snapshots and change tracking by the bitmap driver. I don't understand how you could have completed conversions with shadow copies disabled - sounds like a bug that Converter allowed you to do that at all.
Incremental conversions are tolerant to reboots and software changes, but are not to tweaking the bitmaps tracked by the driver. Antivirus is quite special software - perhaps its re-installation has somehow interacted with bitmap driver.
If "sc query bmdrvr" reports it as running and you still get the error in the log, then it is probably in some bad state.
The safest thing I can think of is to uninstall converter, reboot, verify bitmap driver has been uninstalled, install converter again, and start the conversion from the beginning.
HTH
Plamen