Hola Hansj?rg, it’s been a while!
WordPress Popular Posts has been tested up to PHP version 7.2.13. Haven’t had enough time to try it out under PHP 7.3 yet, however I don’t expect any issues with this version.
I could set up a test site with PHP 7.3 and report back (might take a few days as I’m currently busy with other stuff), or you could just upgrade to PHP 7.3 and see what happens.
If you decide on the latter, should you find any issues with the plugin please let me know and I’ll look into it.
I’ll keep this topic as “unresolved” until there’s a definitive answer as I’m sure other users might be interested in knowing as well.