Hi Thorsten,
thank you very much. Meanwhile I have been able to verify the IDES Backup with the related adjusted shadowservice as well. Only problem is, you need to specify AUTOIGNORE in addition, when the backup was done to a parallel medium.
Just for information for people struggeling along the same path:
If you ever can, use Windows Domain Users and Domain Permissions 
When the backup to the share finishes, it re-writes the permissions of the backup target file. In a non domainsetup, you will loose the permissions for the "local" sidadm. Thus a second backup with OVERWRITE will fail, because the Re-Open is done with the LAN credentials and the target files has only the S-Identifier from the source Server attached. So after the first backup, you need to attach local "sidadm" permissions on the target files to enable the next backup to overwrite.
Finally I have everything working now.
My closing test (may be next week) will be a real restore, which I could not schedule right now.
Thanks again
Volker