Hi, recently I upgraded my forum to 1.4.6. At the same time I changed style to Antiquark by @Mike Creuzer. But then something happened. I went from being ranked as #3/#4 on Google to being ranked as #15. I searched the Internet for a while and found a post with some great tips than pointed me in the right direction. I logged in to Webmaster Tools and found errors related to the mobile style (see picture below).
Im here for any issues concerning ranking, but you should see an improvement moving to our themes as we apply many material design concepts to our themes that Google looks for as well as decrease http requests amongst many other things. As @popowich kindly pointed out I think you are going in the right direction with those page errors. Google may change your ranking whenever you change anything, but if you are doing things better than you were then I imagine you'll come back up in no time.