Error clearing cache

I'm running a simple PHP server locally with php -S 127.0.0.1:80 -t <folder>
Works reliable all the time, but when I click "Clear cache" in the GUI, I get an error:
/admin/cache.json/task:clearCache - no such file or directory

Any idea why that is?

I'm going to remove that bit about using the built in server. It basically is for rudamentary Grav only. It is not able to support many plugins that use more advanced routing: Check the warning here