You really need to tell us all about your setup as we can not look over your shoulder.
What OS?
What version of Calibre (note 5.22.0 had a hot fix that won't notify if 5.22.0 was in place) Download that
Send smaller batches than you have been, disconnect between each batch to allow the Kobo to process them.
By crash: Do you mean Calibre just closes with no message (abnormal ending)?
We really need to see a Log
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