I know you say you've tried everything, but in my experience (and I have been on this forum for a little while) the search engine rarely helps me find just that part of "everything" that I'm actually looking for, even if I'm the one who wrote it. . .

. . .so please don't take this as condescending which I'm not trying to be!
But have you tried the steps I described
here? Admittedly this was on an earlier 5.x.x release but it worked for me and seems to have worked for some others, and I did see the same error you have described before I did it exactly this way.
Do let us know in as much excruciating detail as possible if you still have problems. . .