Backupshare connection issue - access denied without any reason

Hi,
I try to backup my Windows 7 box to my Server 2008 share. Unfortunately sometimes I am not able to access my backup share. Sometimes (!!) ,when I try to setup a new backup, Acronis asks me for username and password (why? The backup share is open for ALL users r/w - ntfs permissions are correct - and the share is allready mounted to a driveletter, that is working fine!). Every user / password were rejected - and all (!! not only the backupshare) on my windows 7 are marked with a red x. When I try to access the shares again within windows explorer - evrything is fine. If I try to access it within Acronis - nothing works. Sometimes a reboot helps - sometimes not.
Can anybody help me ? No other errormessages appeared ... No other application has any issues with the shares!

regards

My config:

- Acronis True Image Home 11 - latest build 6868 -
- Windows 7