Server location can be either a positive or negative ranking signal for search engines, depending on the location of the searchers. While the effect is minimal and applies just in some instances (i.e., not using a ccTLD and not setting geo-targeting in Google Search Console), the server location is important for determining the geotargeting of a website.
Matt Cutts suggests that server location is ranking signal, in this video, and this one.
Q: “Hi, Matt, could you confirm whether the geographic location of the web host has any significant ranking factors for organic SEO?”A: Yes, it does”Q: “What impact does server location have on Google rankings?A: … back in 2000-2001 type time frame, we started to look at where is a server located, it’s IP-address.”
“Google generally uses the following elements to determine the geotargeting of a website (or a part of a website): […] Server location (through the IP address of the server) is frequently near your users” source
“Because we attempt to serve geographically relevant content, we factor domains that have a regional significance. For example, “.co.uk ” domains are likely very relevant for user queries originating from the UK. In the absence of a significant top-level domain, we often use the web server’s IP address as an added hint in our understanding of content.” source