Hi, this happened to me as well just a few days ago. I had just put on new props and was thinking it was something to do with them.
I have a balancing ring but took it off due to me using an ND filter. Any input would be great.
Weather and holiday have held me up; but from the links provided above there are some steps that can be done to 'troubleshoot' the issue, I hope to do all of them this weekend and see if i get a positive result.