I had this same problem today. My Unraid server had been shutdown for the last 6 weeks. Logging back in from the webGui was not a problem. While the server was offline my Win10 mapped drive showed a red X. This was normal since the server was not online. However, once the Unraid server was accessed and the array was started, this did not automatically resolve the red X for the mapped drive. Similar to the OP, I could not access the network server, either. The Windows Security window would open and ask for a userid and password. This confusing. Here is what corrected my problem, easily:
Originally, in Unraid Settings\SMB Settings\SMB Extras, I had already created the standard script for Samba Extra Configuration. Here is that script:
[rootshare]
path = /mnt/user
comment = for a single map network drive share
browseable = yes
valid users = bmoney
write list = bmoney
vfs objects =
This is the name of the mapped drive that I had created in Win10. The problem was that I could not get the red X to clear or reset. So, I simply "disconnected" the mapped drive which was listed as rootshare (\\myserver) (S:). Then, I went back and remapped the same \\myserver connection. The rootshare appeared and I connected to it. By doing this, I was able to see and access all the file associated with the Unraid "rootshare" directories and files. But, also, this directly enabled the Network connection to "myserver" without the earlier Windows security screen opening.