@lelelele Welcome to the community
You can read this topic for why you are seeing this problem now:
Not all sites or clients that use Lets Encrypt certs are affected. The ones affected are mostly poorly configured or older versions. Servers have the option of sending various certs and chains.
Better advice could be given if you provide a specific example of a curl that is failing.
But, a very rough guess is that your headless Debian does not trust the ISRG Root X1 certificate. You could try updating your CA certificate store. I am sorry but I am not familiar enough with Debian to describe that.