Rerunning the dsync command right after the timeout (see previous message) gave me these errors:
dsync-local(vmail): Error: remote: dsync-remote(vmail): Error: Corrupted dbox file /home/.incoming_mail_migrations/users/user%domain.com/mdbox/s torage/m.60 (around offset=807789): msg header has bad magic value dsync-local(vmail): Error: remote: dsync-remote(vmail): Error: Corrupted dbox file /home/.incoming_mail_migrations/users/user%domain.com/mdbox/s torage/m.60 (around offset=807789): msg header has bad magic value dsync-local(vmail): Error: remote: dsync-remote(vmail): Warning: mdbox /home/.incoming_mail_migrations/users/user% domain.com/mdbox/storage: Inconsistency in map index (2,75904 != 2,82524) dsync-remote(vmail): Warning: mdbox /home/.incoming_mail_migrations/users/user% domain.com/mdbox/storage: rebuilding indexes dsync-local(vmail): Error: remote: dsync-remote(vmail): Error: Corrupted dbox file /home/.incoming_mail_migrations/users/user% domain.com/mdbox/storage/m.60 (around offset=807789): msg header has bad magic value dsync-local(vmail): Error: remote: dsync-remote(vmail): Warning: dbox: Copy of the broken file saved to /home/.incoming_mail_migrations/users/user% domain.com/mdbox/storage/m.60.broken dsync-local(vmail): Error: remote: dsync-remote(vmail): Error: Corrupted dbox file /home/.incoming_mail_migrations/users/user% domain.com/mdbox/storage/m.60 (around offset=807789): EOF reading msg header (got 0/30 bytes) dsync-local(vmail): Error: remote: dsync-remote(vmail): Warning: mdbox /home/.incoming_mail_migrations/users/user% domain.com/mdbox/storage: rebuilding indexes dsync-local(vmail): Error: remote: dsync-remote(vmail): Error: mdbox /home/.incoming_mail_migrations/users/user% domain.com/mdbox/storage: Duplicate GUID 1341304555.M433317P26661.n13,S=142447,W=144391 in m.62:790375 and m.60:790375