Thanks Scott, I did find the LibSM tweak on the MRMC forum and that fixed it. I never thought of it because I thought SMB was SMB, and I'm using whichever version apple includes in high Sierra.Which version of smb is your server running ? smb1, smb2 or smb3 ? Did you try switching to libdsm (in smb client settigns) ? We are working on bringing in a new smb lib.
If you are having issues, don't suffer in silence, register on our forums and make a post about your issues. The crystal ball is missing, someone stole it and we only know about issues if users report them.