Resolved issues with OpenVPN authentication and certificate validation.
Netgate occasionally rotates archives. If the direct link fails: pfsense 251 download iso link
https://files.netgate.com/mirror/downloads/ pfsense 251 download iso link
For (embedded/CF card) users:
Netgate (the parent company of pfSense) maintains an official archive server. The direct path for legacy releases is typically: pfsense 251 download iso link