Hello,
Check if the "kanzlei" folder is indeed where your 400,000 documents are stored. If not, add the correct folder path to be indexed. Ensure no subfolders or file types within the "kanzlei" folder are excluded from indexing. Check the 'Advanced Options' in Indexing Options for any exclusions set at the folder or file extension level.
Ensure that the user account under which the Windows Search service runs (usually Local Service or Network Service) has read permissions for all the files and folders within the indexed locations. Right-click the folder, select Properties > Security > Advanced, then check the permissions for these service accounts. They should have at least 'Read & Execute', 'List Folder Contents', and 'Read' permissions.
Also try to reset the permissions on the Windows Search directories. The default location is %SystemDrive%\ProgramData\Microsoft\Search\Data. Grant 'Full Control' to the SYSTEM, Administrators, and the relevant service account (as mentioned above).
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