Are both load balanced web servers sharing the same database, or does each server have it's own database?

One thing I would suggest is if you can get a direct route to one of the servers (either DNS entry or the IP address of the server) and point your mirroring job at that rather then the shared URL/IP and see if that works. If the load balanced servers are sharing a database, mirroring to one of the servers will get the data to both of them, though you'll need Remote Events enabled.

I'm experiencing a similar issue uploading to one server in a load balancing pair with shared database.

The first job sometimes fails but the subsequent one will always succeed. 

I've set the job up with a different server name AND port so there's no way the load balancer is getting involved.

same error as above:

Failed to call StartChunkPackage with context2d517044-a20d-4161-81a8-736e51a98969 becuase there is not Header info in Mirroring job.

I can confirm that this is not to do with the load balancing as it returns the same error even when the load balancer is switched off.

Can anyone from EPiServer confirm what causes this error?

I get the following entries in my log file...

2012-10-08 11:00:54,800 DEBUG [TransferingThread] EPiServer.MirroringService.MirroringTransferProtocol.Common.TransferQueueManager.HandleStatus - Context = 2d517044-a20d-4161-81a8-736e51a98969 State = TransferingError Starting Errors Failed to call StartChunkPackage with context 2d517044-a20d-4161-81a8-736e51a98969 becuase there is not Header info in Mirroring job
End of Errors 
DestinationRoot=3  ,DestinationPath=C:\Windows\TEMP\episerver\cms6\mirroring\targetfiles\2d517044-a20d-4161-81a8-736e51a98969  ,DestinationPageGuid=3dba82ce-1a4a-446a-b8f1-b67f0e8e4b64  ,TypeOfData=Page  ,SourcefilePath=C:\Windows\TEMP\episerver\cms6\mirroring\sourcefiles\2d517044-a20d-4161-81a8-736e51a98969\P2012108_10-0-37-971_0.TMP  ,TargetfilePath=P2012108_10-0-37-971_0.TMP  ,FileSize=210732  ,SizeOfSentData=0 ,SizeOfReceiveData=0

2012-10-08 11:01:03,082 ERROR [58] EPiServer.MirroringService.MirroringProviderBase.ErrorHandling - Context = 2d517044-a20d-4161-81a8-736e51a98969 State = TransferingError Starting Errors Failed to call StartChunkPackage with context 2d517044-a20d-4161-81a8-736e51a98969 becuase there is not Header info in Mirroring job
End of Errors 
DestinationRoot=3  ,DestinationPath=C:\Windows\TEMP\episerver\cms6\mirroring\targetfiles\2d517044-a20d-4161-81a8-736e51a98969  ,DestinationPageGuid=3dba82ce-1a4a-446a-b8f1-b67f0e8e4b64  ,TypeOfData=Page  ,SourcefilePath=C:\Windows\TEMP\episerver\cms6\mirroring\sourcefiles\2d517044-a20d-4161-81a8-736e51a98969\P2012108_10-0-37-971_0.TMP  ,TargetfilePath=P2012108_10-0-37-971_0.TMP  ,FileSize=210732  ,SizeOfSentData=0 ,SizeOfReceiveData=0

I have the same issue, no load balancer... EPiServer are investigating it...

Cool, thanks for letting me know. I am having some VERY high CPU % in relation to this problem, which results in needing to recycle the application pool. Is that another symptom you are experiencing?

No, not really.. well I am getting high CPU usage, but guess that's just the mirroring job doing it's thing. Both cores seems to be working at about 50% when the mirroring job is creating the package.