I have little knowledge about Windows, but I know that the SYSTEM account
is usually related to processes running as services.

SYSTEM has restricted rights on number of things, despite appearing as a
'super-account', an attempt from Windows to mitigate services
'super-powers'.
It would not surprise me if SYSTEM was not authorized to access mapped
drives.

Try changing the user executing the service or try running nginx as a
normal user process.
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*B. R.*
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