[SOLVED] Mailbox import requests stuck in 'queued' state

We have a small Exchange 2013 environment with one server having all roles. I try to import archive mailboxes from an Exchange 2010 environment. It has worked quite well for a while. However, all import requests are now 'stuck' in 'queued' state. Restart of Replication service, transport service and even server reboot didn't help.

When I run

get-mailboximportrequest | Get-MailboxImportRequestStatistics -Includereport | fl 

I observe that PositionInQueue value tells me that there are 85 import requests waiting :

ItemsTransferred                       :
PercentComplete                        : 0
CompletedRequestAgeLimit               : 30.00:00:00
PositionInQueue                        : 85/90 (Position/Queue Length)
InternalFlags                          : None
FailureCode                            :

There should be no requests waiting as I removed all requests in advance before I issued new import requests:

get-mailboximportrequest | Remove-MailboxImportRequest

How can I really reset an import queue by zeroing PositionInQueue ?

Any suggestions/tips?