I have come back to my site for which I did not do any SEO since a few years. First thing I noticed was that Google did not consider it mobile friendly anymore. So I updated the mobile template and now Google seems to like it. But right after I did that there happened a significant drop in the site's rankings for all important keywords and for all platforms (desktop, mobile, tablet). Accordingly the visitor numbers have dropped significantly as seen in the graph below.
The only thing I did apart from the mobile friendly template was to register a free account with a Google rankings checking service. I have no idea why my rankings have dropped. This can't be a coincidence, the date coincides exactly - around 20 October. I've even got a weird idea that the ranking service I subscribed to might have done something to force me buy their paid plan. This sounds really weird by I don't see any explanation more logical than that.
Does anybody have a better idea?