@substring got it working, we simply have to add "vers=2.0" (or 2.1 / 3.0 /3.02) to the mount commands, i.e:
sharenetwork_smb1=ROMS@192.168.0.2:RecalboxRoms:username=roms,password=roms,vers=2.0
sharenetwork_smb2=SAVES@192.168.0.2:RecalboxSaves:username=roms,password=roms,vers=2.0
1.0: Win 2k/XP/Server 2003/Server 2003 R2
2.0: Vista SP1/Server 2008
2.1: Win7/Server 2008 R2
3.0: Win8/Server 2012 (w10?)
3.02: Win8.1/Server2012 (w10?)