It sounds like you may have applied the C2 level security fix for 'restricting null session access over named pipes', which (I think) can only be done prior to creating shares. If you check the following reg entry on this box and compare to another box you may find some difference:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters]
"NullSessionShares"=hex(7):
Where "hex(7):" = Restrict.
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