Linix Mint 19.1 Blender beta 10.1.19 loop select (ALT left-click) not working
this is not a problem of blender...
on mint- cinnamon preference pannel select the icon "window" and put "super" key instead of "alt" key
I think @noki paike (amonpaike) said it all, please alter your OS preference.
Will close this as resolved (but feel free to comment again if issues persist...)