It was the default rule that it started with. Mail from a specific person (email) moves to a specific folder. I gave it a blah@blah.com email to a random folder in my list and called it "Test Filter" and saved it, and it immediately errored.

It appears that this issue was due to a bug in eM Client 8 BETA. We have been communicating with them regarding this issue, and they just let us know that they have fixed it internally and to expect the fix in the next build.

Thanks Andrew!