Many security_mode=share issues here, what's the problem?

So further:

As my Windows backups failed, I renamed, on the NAS the 'WindowsImageBackup' directory' to *old, so that the backup could have a new empty target.

So good for a while. New backup is running, taking forever as it is a new full backup...

Then hours later...Windows backup errors 0x81000037

See Microsoft

These issues occur because the folders or libraries that you are trying to back up or restore include a reparse point. Reparse points resemble Windows shortcuts or to Unix symbolic links. A reparse point may be used to point to a file that is temporarily relocated on a different drive. For example, a reparse point would allow a folder such as C:\DVD to point to Drive E, the actual DVD drive.

Issue 2 This issue occurs if the reparse point is a mounted volume that contains compressed files (for example, multimedia files, .zip files, and so on).

Work around issues that occur when you try to back up files Issue 1 and Issue 2 To work around these issues, remove the reparse point from the library, and then run the backup wizard again. To back up content for this reparse point (in this case, the FAT volume or the mounted volume), select the absolute path of this location in the Windows Backup configuration user interface. To locate reparse point folder or its subfolder, follow these steps: Click Start button, type CMD in the Search program and files box and then press Enter. In the Command Prompt window, type DIR /AL /S and then press Enter. The list of JUNCTION (one type of reparse point) list is displayed. Note You see several entries that say "junction" and the location that the entries point to (“junction point” is another name for “reparse point”). When a program tries to write to a directory that is really a reparse point, those files are sent (completely blindly to that program) to the other directory instead. To delete mounted volume reparse point, follow these steps: Locate the reparse point you found, right click the reparse point icon and click Properties. In the General tab, confirm the folder Type is Mounted Volume and then delete this folder.

Continued....

EDIT Backups had been working weekly, without issues, before the USER mode 'fix' /EDIT