Since few days, I have some podcasts that were perfectly working that cannot be played anymore due to certificate non validated.
It seems that one of the certificates has changed 4 days ago and that gnutls is not managing it properly.
Here the check on the ssl certficate for one of the concerned URL :
Related post that explains the issue :
So I think there is a need to do some updates on volumio to handle this otherwise we will have to wait for all servers using this certificate to hopefully fix it.