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Hi @maprins,
Google is complaining about the color of your links not having enough color contrast against the white background. This blue color is coming from your theme (GeneratePress), not from WordPress Popular Posts.
To fix this problem you can:
A) Update your theme’s stylesheet default color for link tags so it has a better contrast ratio, or
B) Add the following CSS code to your site so your popular posts link color is darker and thus passes the contrast ratio test:
.wpp-post-title {
color: #2f4468 !important;
}