Issue with updateItem readStatus

I (or rather a customer) have a strange issue with an inbox folder.

Our software retrieves a message from a delegate inbox (successfully with a getItem call), then follows up with UpdateItem to mark the message as read.

The server replies with: ResponseMessage "Access is denied. Check credentials and try again"
(calls on the users own inbox succeed, only calls on delegate accounts fail)

As far as i understand you need only read permission on a folder to change the read status. But even if we change the users permissions to owner the error message persists.

The UpdateItem operation is successful in other folders, like Contacts or Notes (on the same delegate account), only the read status change does not work.

The xml of the call:

<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:soap-enc="http://schemas.xmlsoap.org/soap/encoding/"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:n1="http://schemas.microsoft.com/exchange/services/2006/messages"><soap:Header><n2:RequestServerVersion xmlns:n2="http://schemas.microsoft.com/exchange/services/2006/types"
    Version="Exchange2007_SP1" /></soap:Header><soap:Body><n1:UpdateItem ConflictResolution="AutoResolve"
    MessageDisposition="SaveOnly"><n1:ItemChanges><n2:ItemChange xmlns:n2="http://schemas.microsoft.com/exchange/services/2006/types"><n2:ItemId Id="AAMkAGYwMzQ4ZmY0LWFhZGItNDIxMC05MmRlLWQ3MTViMmU4M2EyYQBGAAAAAABq50hXlBeeTrlO/SxN2QYsBwCfly/XhdlaQKuxj6sImPdgAAAAT1GFAABDBLQY/DhbSYMkbaSRh57+AAB0HoKaAAA="
          ChangeKey="CQAAABYAAABDBLQY/DhbSYMkbaSRh57+AAB0HpKa" /><n2:Updates><n2:SetItemField><n2:FieldURI FieldURI="message:IsRead" /><n2:Message><n2:IsRead>true</n2:IsRead></n2:Message></n2:SetItemField></n2:Updates></n2:ItemChange></n1:ItemChanges></n1:UpdateItem></soap:Body></soap:Envelope>