[Dovecot] dsync conversion error

Hello When converting mailboxes I get that kind of error messages :

dsync(xxx): Error: Next message unexpectedly lost from mbox file /user/xxx/mail/Sent at 9580036 (cached)

the dsync command I run is

dsync -u xxx mirror mbox:/user/xxx/mail:INBOX=/var/mail/xxx

Dovecot mail_location configuration is :

mail_location = mbox:~/MailSpace/Mailboxes:DIRNAME=Mbox-Messages:INDEX=~/Index:CONTROL=~/Kontroll

Note that ONLY happen on users's "Sent" folder, never on any other folder

Thanks for any advices.