I believe the issue you are facing is addressed in the SUM guide chapter 5.1
5.1 Meeting the Requirements for the SUM Directory
https://websmp110.sap-ag.de/~sapidb/011000358700000783582011E/sum_abap_windows_oracle.pdf
5.1 Meeting the Requirements for the SUM Directory
If you do not specify a path when you unpack the Software Update Manager package, the SUM directory is automatically created as a subdirectory of the directory where you unpack the software package. However, we recommend that you use the standard path <DRIVE>:\usr\sap\<SAPSID>.
Note
If a directory with this name already exists, it has to be empty. Otherwise the startup of the update fails.
If you use for the SUM directory another location than the standard path <DRIVE>:\usr\sap\<SAPSID>, you have to copy the access control list (ACL) from the <SAPSID> directory to your SUM
directory. For this, you open the powershell console with elevated rights and enter a command with the following structure:
set-acl [SUM directory] (get-acl [<SAPSID> directory]).
Example
Your SUM directory is located in D:\SUM.
Your <SAPSID> directory is located in C:\usr\sap\SID.
You enter the following command in the powershell console with elevated rights: set-acl D:\SUM (get-acl C:\usr\sap\SID)
Caution
● Do not use symbolic links to remote directories.
● Do not use volume mount points.
● Set security settings even for local symbolic links or junctions. If you use for example the SUM directory D:\SUM and a symbolic link to C:\usr\sap\SID\SUM, you enter the same above-mentioned command in the powershell console.
● Alternatively, you extract the Software Update Manager under the <SAPSID> directory.
Source : SUM Guide 1.0 SP11
Regards
Reagan