All of a sudden, for me, my SMB shares stopped working too. Operation not permitted was returned.
After pulling out the last two hairs from my head I determined it to be an SMB version issue. Capped the max SMB version to v2 and was able to browse SMB shares on my Synology again.
The confusing thing is everything was working fine for ages prior to this popping up.
Now, I did make some security changes on the Synology in response to security concerns about ransomware attacks. But I never touched SMB version settings. it was and still is set to use SMBv1 though v3.
This issue popped up on both my x64Generic and OdroidC2 installs.
Thanks for feedback AG. I had to set SMB1 (previously 2). Not sure what 's up here ...