I've been suffering from intermittent network folder access deny since upgrading to 2.4.X. Not referring to any particular folder, but just in a rather random pattern. This time folder1 was locked, another time, folder2 was locked .... and so on.
To resolve the problem, I can either stop/restart the samba service or reboot the nas.
My client PCs are running Win 7 and Win 10.
Absolutely no such problem when running 2.3.X ... ultra stable!