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Go with Serpbook or rankchecker...I was furious to receive a report from Moz today telling me that around 50 keywords went from Rank 1 to rank nowhere. after they brought me back from my heart attack incident (sarcasm) I decided to check SErobooks and a free tool called rankchecker and my keywords were fine and where they should be ranking wise. I do not trust Moz on rankings at all tbh

Thank you Keri for coming back to clarify the situation. Richard, here's a useful link you may want that can help you fixing the error 406. Hope it helps. Good luck.