When attempting to save page in Admin plugin I get an Invalid Security Token error message and page is not saved. I’m running the latest v1.1.0 rc3 version. I get this message when using Edge in Win10 on private network (not behind proxy). I tried clearing browser cache - still the same. I changed to Chrome and performed the same action - no error. It seems that it is Edge related.
I only have this when I try to save a page after a long period during which I do nothing. My session is “expired” I think.
Could it be this in your case ?
My 2 cents,
Maybe create an issue here? https://github.com/getgrav/grav/issues
Please make sure you say that it’s only MS Edge browser.