Mailbox move stuck

Hello,

I'm trying to move mailboxes from Exchange 2007 to 2013. The move request seems to be stuck in the "Syncing" mode.
Exchange 2007 environment consists of CAS and MBX servers, Exchange 2013 has both roles on the same machine.
Mail traffic between servers, to/from internet works without issues.
Where would I start to troubleshoot it?

Thanks.

Memento Mori