You have to Build that Plugin from source against the UE4 version you are running (Source or Launcher). If the Plugin Provider supplied you with Binarys for 4.18 than you are fine on the Launcher Version. If you just use the 4.17 ones it won´t work.
Huh. Looking at the source Code right now. It gets to the Point where it definitly knows that it has to load the plugin from the discriptor but but the Find afterwards returns a null pointer. The Question is why its not Loaded even though Binarys are there and the Descriptor seems fine too at that Point.
Wild guess but did you Compile the Binarys for a Shipping Build? Does it work with other Build configurations? Did you find anything in the Package log related to your plugin? And last Question does your plugin come with Source or do you just provide the Binarys?
But it sounds indeed like some strange bug with the Conditions provided (especially the has to be the only plugin one makes no sense to me)