Live VDI migration fails with dd_sparse error and 'The VDI copy action has failed'

We attempted to migrate a large VDI from SR to SR online. Same attempt for cross pool migration was done.
Alas, after 6 to 7 hours of running, it fails with 'The VDI copy action has failed'. Attempted ~5 times to check.
VDIs taking less than 6 hours to migrate are migrating OK. Same happens for cross-pool VM migration.

IMPORTANT UPDATE: Seems like it was related to specific VDI and problem is gone with coalescing (exactly this VDI contained three VHDs and had to be coalesced). So may be not timeout related, but something else.