After upgrading to Linux Mint 19.3 and Krusader 2.7.0, I could not evoke Krusader Root Mode. It just stayed in a loop asking for my sudo password. The error mentioned the kdesu could not authenticate me.
I tried making a symlink pointing kdesu to kdesudo – no help.
Searching Synaptic (the real Ubuntu one, not the crippled Mint one), I found and installed libkf5su-bin, “runtime files for kdesu” – and now I can use Root Mode Krusader!
Apparently, I got libkf5su-bin via a PPA, not the default one in the Mint/Ubuntu repository, however, the default one works just fine as well.