Hi ,
How did you access A's shared folder from C? Via \IP address or \hostname? Use both \IP address and \hostname to do a test.
If C is accessing A' shared folder. Then C acts as SMB client and A acts as SMB server. Go to C's SMB client log and A's SMB server log to check if there is something related for us to troubleshooting.
In addition, try to manually add username/password for UNC path in windows credential manager, see if it can work.
Best Regards,
Candy
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