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Thank you for helping. It started well before that actually, more or less in April / May 2017, but it did get worse in October 2017 (although based on your image one of the competitors improved its backlinks right after that). Well I'm sure it had something to do with it. Unfortunately my client doesn't have the same budgets for referral / authority building as some of the competition does.

I read about "Fred" since Moz mentioned it in their newsletter yesterday. Two interesting links:

https://www.buzzfeed.com/leticiamiranda/overstockcoms-search-engine-business-is-crumbling-after 
https://www.searchenginejournal.com/google-algorithm-history/fred-update/ 

I guess this sounds familiar and aligns with the period that my client suffered. I don't remember ever reading anything about it, but it might help.