There´s info on the bottom about functionality thru versions...
"Note: plugin is written to work with R16 upto R20. Not tested on R21/S22. Will not work with R23 (yet). "
with loop is meant you select just one edge and plugin will select rest of edges in the loop. When you select loop and tell plugin select loop, maybe plugin go in circles and die
I have placed plugin into preference "plugins" folder without any plugin path in preference and load plugin correctly without issues.
As Daniel said, console say : RingLoop v0.5 - ready