Request for token getting TrustFailure error

Hi All,
I have some Mono apps running on RaspberryPis for years and suddenly overnight they can’t authenticate with Auth0. The request returns a “TrustFailure” error message.
The same C# code runs fine on my Windows 11 device, which makes me wonder if its a certificate issue, but there are no certs installed in my RaspberryPi devices as far as I know.
Can anyone point me in the right direction for a solution ?

Many thanks,
Damien.