Ok, found out that:
- https://yamaworld.fr:5232 works with Firefox/27 on my PC, but
- https://yamaworld.fr:5232 doesn’t work with Firefox/27 or the default browser on our Galaxy Note 10.1 (Android 4.1): “untrusted issuer”

So let’s have a look at the certificate chain with OpenSSL:
# openssl s_client -host yamaworld.fr -port 5232
---
Certificate chain
0 s:/OU=Domain Control Validated/OU=Gandi Standard SSL/CN=yamaworld.fr
i:/C=FR/O=GANDI SAS/CN=Gandi Standard SSL CA
---
If I look in Firefox on my PC, I see this chain:
- UTN-USERFirst-Hardware (serial nr. 44:BE:0C:8B:50:00:24:B4:11:D3:36:2A:FE:65:0A:FD)
- Gandi Standard SSL CA (5A:B6:1D:AC:1E:4D:A2:06:14:C7:55:3D:3D:A9:B2:DC)
- yamaworld.fr (00:DC:BB:78:54:7E:47:7C:CF:61:72:62:DE:B7:91:0F:E9)
So the problem is that your server doesn’t send the the Gandi Standard SSL CA as an intermediate certificate. On devices where the Gandi Standard SSL CA is available by default, it works, but on Android devices, the intermediate certificate is not in the trust store by the default. However, UTN-USERFirst-Hardware is available:
.
Configure your Web server to send the Gandi Standard SSL CA as an intermediate certificate
and it will work. Apache FAQ