Hi,
if you only have one version of Unity on your computer, then it's not a issue with the cache.
I did start a project using 2018.1 and I must say I experience lots of crashes too and they were not related to PlayMaker, like random crash on asset import, random crash of lighting tweaking, I restart at least 4 5 times during a work session...
so I am afraid 2018.1 is just not there yet and we shoudl wait for 2018.2 for production projects...
so I can confirm Playmaker do work with 2018.1, on your end is it 100% reproduceable, that is you just can not create a project and add Playmaker at all?
have you tried to disable unity analytics? maybe this will help, when you create a new project you can decide to not send editor analytics for that projects and so unity will not try to contact its server constantly. Could help.
Bye,
Jean