Thanks for confirming.
What happens if you change these to include the drive letter and enclose the values in quotes?
server.ssl.certificate: "c:\kibana-8.5.3\config\kibana-server.crt"
server.ssl.key: "c:\kibana-8.5.3\config\kibana-server.key"
If that doesn't work, then what happens when you also escape the slashes?
server.ssl.certificate: "c:\\kibana-8.5.3\\config\\kibana-server.crt"
server.ssl.key: "c:\\kibana-8.5.3\\config\\kibana-server.key"