Vibe.d error

[JG]

Hi,

We are intermittently getting the following error:
Accept TLS connection: server
OpenSSL error at ../ssl/record/rec_layer_s3.c:1543: 
error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert 
certificate unknown (SSL alert number 46)
HTTP connection handler has thrown: Accepting SSL tunnel: 
error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert 
certificate unknown (336151574)
Full error: 
object.Exception@/home/jg/.dub/packages/vibe-d-0.9.3/vibe-d/tls/vibe/stream/openssl.d(578): Accepting SSL tunnel: error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown (336151574)
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Anyone have any idea what might cause this?