Our Sophos anti-virus at work blocked access to every website this morning because of the leap year, had to disable the web filtering to get us back up and running.
The most likely thing to me would be that it attempted to issue a 1-year-long certificate, but naively produced a date of Feb 29, 2025.
Dates in certificates are encoded as `YYMMDDhhmmssZ` so it would certainly be possible to produce a certificate with an invalid date.