I'm unsure if this is a bug or a wrong server configuration, but with Matomo 3 everything worked.
Currently I am using Matomo 4 RC2 with PHP 7.4 and nginx 1.19.4 which is working mostly fine.
But, the graphs stopped working. I still see visitors logged, but the graphs indicated that there are no visitors. Also the statistics are empty.
If you need more information I would really like to help making Matomo even better! :)