I can confirm this behavior with openSUSE Leap 15.0 (Plasma 5.12.5, Frameworks 5.45.0) on the client and Samba 4.6.14 on the server.
Dolphin shows nothing with smb:// if I go to the share directly in Dolphin it works. With smbtree the shares are showing correctly.
The following file on the client helps, thank you: ~/.smb/smb.conf [global] client max protocol = NT1
But it would be better do solve this issue in the smb kio plugin directly.
I can confirm this behavior with openSUSE Leap 15.0 (Plasma 5.12.5, Frameworks 5.45.0) on the client and Samba 4.6.14 on the server.
Dolphin shows nothing with smb:// if I go to the share directly in Dolphin it works. With smbtree the shares are showing correctly.
The following file on the client helps, thank you:
~/.smb/smb.conf
[global]
client max protocol = NT1
But it would be better do solve this issue in the smb kio plugin directly.