I've been having this problem too since upgrading from 4.6.6. to 5.1.0. Have you upgraded too?
Is still the task running after restarting the ePO services? If yes then please open a case with support.
Is there another task running? maybe that task is locking the repository, which can cause the other tasks to wait until the repository is unlocked again.
Maybe a replication task, or the file / folder structure of the repository became corrupted..