I just solved the issue.The problem was that mosquitto was not capable of reading the files not because of permission issues but because of the filepaths.
So the thing was that when defining the filepaths in the mosquitto.conf I had to use:
cafile /etc/mosquitto/ca_certificates/ca.crt
certfile /etc/mosquitto/certs/server.crt
keyfile /etc/mosquitto/certs/server.key
Instead of:
cafile c:\etc\mosquitto\ca_certificates\ca.crt
certfile c:\etc\mosquitto\certs\server.crt
keyfile c:\etc\mosquitto\certs\server.key
solved Error SSL/TLS connection in MQTT with mosquitto broker