“License server unreachable” – fixed by running DSLS as Admin and disabling IPv6 on the loopback adapter (Control Panel > Network > Adapter settings > Disable IPv6 on localhost).
For those with a valid license server file: install+catia+v5+r21+on+windows+11+updated
: Ensure you have administrative privileges to modify system files. “License server unreachable” – fixed by running DSLS