Running OpenSUSE Tumbleweed w/ Plasma 5.10.5 and Frameworks 5.37 and am unable to access my Samba server with Dolphin using SMB3, as I have set my client to use max protocol = SMB3. Gives error "Internal Error Please send a full bug report at http://bugs.kde.org Unknown error condition in stat: File exists". I notice that the username password box doesn't pop up anymore, just gives that error. Tried Thunar just to be sure and it works fine using the SMB3 protocol. NT1 still works, at least until CentOS 7.4 comes out then as I understand it all the old protocols are getting phased out. Not sure what other info to include here as I am unable to find these errors in any logs.
I have the same issue (same error message) on Arch Linux with Dolphin v17.08.1 Dolphin is able to list my shares but any attempt to actually connect fails, while it works fine with thunar.
I also have major problems with SMB in dolphin and kio-extras 17.08.1-1. Some shares I can't connect to at all and some only work partially (in some folders). I believe the authentication of shares could be the problem?
I also see this with Fedora 27. I can list the shares on a samba server, but can't access any content on them.
Oh - and I'm not quite sure if it is related, but I do not have anything configured for 'max protocol' on either the client or server side.
I can confirm this on 17.08.2
I have tried to downgrade both kio and dolphin to version 16.08.3 (I saw on a friends Debian Stable machine that it still worked). That didn't help at all and the same error ("File exists") still came with every reload. Then I tried downgrading the packages "samba", "smbclient" and "libwbclient" (Arch Linux) to version 4.5.3-1 and what a surprise: it worked. I didn't try yet which version broke it though.
According to bug 339752 comment #7 it was changed in smbclient 4.7.
*** This bug has been marked as a duplicate of bug 385708 ***