But on Ubuntu 18.04 and on LibreElec 8.25, when I browse my workgroup for SMB shares, I cannot see any samba servers, even my windows pc.
Kodi + LibreELEC now default to SMBv2 (and up), and SMBv2+ no longer supports network browsing, so you will need to add your sources manually via "Add a network location...". SMBv2 also requires a user account with full credentials, meaning a user with a password. Seeing that you use Ubuntu, that shouldn't be a problem.
SMBv1 is a basically a Swiss cheese when it comes to network security. Google for 'WannaCry' if you want to see the outcome of that.
XP is very unlikely to have SMBv2 support anyway. The 7 and 10 versions will probably have been upgraded to a default SMBv2 by now.
You can change the minimum SMB protocol in the LibreELEC Settings Add-on to SMBv1, and even choose the legacy option.
Just don't come to us when a sudden network intrusing has taken place afterwards.
