Just wondering if you can disconnect DEF tank inlet hose to pump/filter, and disconnect DEF doser attached to 1 box. And run a inlet line from pump/filter to bucket of hot/warm water. Use software (my case Detroit dddl 8.13) and run the quantity test 1 to several times to clean entire system of def crystals. Maybe even use ionized or RO water. And allow to sit a bit in between times running quantity test to allow crystals to desolve. Then run good DEF to flush out water. Cost a filter and copper seal at doser/1 box.
Reset any codes, if applicable, caused by process.