to put quite simply, there's something wrong with your clutch. Is this issue when the cover is on too? you might just have the clutch cover pressing the starter bll onto the clutches. Possibly you're missing some washer or spaces and jamming things together - particularly behind the clutch bell, that could bind against the case. Also if your spacers or spring washers or preload is off, it'll bind.
also also, try just lubricating everything and cleaning it all well. sounds like it's missing assembly bits in there tho honestly
It's also sometimes possible that the pedal gear engagement is not in it's slot - a common failure if it's been rebuilt - and that can lock up the drive, can you turn the main gear behind the clutch while the clutch is out? Sounds like yes from your descriprtions