This probably needs narrowing down or finding better info on the failure.
Windows appears to put Connectivity events in event log below, which you can view as an Administrator:
Windows Logs --> Applications and Services Logs --> Microsoft --> Windows --> SMBClient
You can look at all the logs there to look for any information on the error.
If you have a different SMB server (even Windows), does it work better?
Because this appears sensitive to the account, testing it as SYSTEM in a human-friendly way might help.
PsExec run from an Administrator account will let you run many things as SYSTEM, but not File Explorer.
Something like psexec -s -i cmd.exe runs Command Prompt. Run whoami to verifyy system account.
From there you see if things like net use work, and if they don’t, do they (or event log) report their issues?
powershell.exe is reported to do UNC well, unlike cmd.exe, so maybe try something like this as SYSTEM:
Get-ChildItem <UNC path>