MSA fails to copy messages with "-2146644475 Unknown Error" (4266138)

The following entries can be seen in the MSA (Mail Source Agent) log:

9/6/2011 12:55:58 AM MailKernel::LogStatus TraceMsg 2040 Folder 'Transfers', sync type: [0x10 - Hidden messages], steps 1 of 1 done, sync aborted - False.
9/6/2011 12:55:58 AM MailKernel::LogStatus TraceMsg 2040 Folder 'Transfers', sync type: [0x20 - Messages], steps 2 of 1141 done, sync aborted - False.
9/6/2011 12:56:19 AM MailKernel::LogStatus TraceMsg 2040 Folder 'Transfers', sync type: [0x20 - Messages], steps 115 of 1141 done, sync aborted - False.
9/6/2011 12:56:39 AM MailKernel::LogStatus TraceMsg 2040 Folder 'Transfers', sync type: [0x20 - Messages], steps 229 of 1141 done, sync aborted - False.
9/6/2011 12:57:10 AM MailKernel::LogStatus TraceMsg 2040 Folder 'Transfers', sync type: [0x20 - Messages], steps 343 of 1141 done, sync aborted - False.
9/6/2011 12:57:37 AM MailKernel::LogStatus TraceMsg 2040 Folder 'Transfers', sync type: [0x20 - Messages], steps 457 of 1141 done, sync aborted - False.
9/6/2011 12:57:46 AM CFolder::Synchronize Error -2146644475 Unknown Error. (Microsoft Exchange Server Information Store) File: 'aeMAPIFolder.cpp' Line: '560' (Folder: 'Transfers', SourceKey: 'E5DE93388480A24D8A585C6827729A6600000046ECC7')
9/6/2011 12:57:46 AM MailExport::SynchronizeContent Error 2032 Cannot synchronize messages in the folder '\Inbox\Transfers'. Messages type 32.