Server side rendering is supported now. See this test repository for working examples for Astro, Next.js, Remix and Vite:
https://github.com/grubersjoe/react-activity-calendar-tests
Addtionally, this package also exports <ActivityCalendar /> as named export now. In some environment it seems crucial to use the named export instead of the default one. The default export will be removed in the next major version.
// Please do this
import { ActivityCalendar } from 'react-activity-calendar';
// Instead of this in future
import ActivityCalendar from 'react-activity-calendar';