We are seeing some Diffie-Hellman public key errors when accessing some of the sandbox CUCM and CUP servers. The issue is mainly occurring with chrome and Firefox. We apologise for this and currently looking at a solution.
In the meantime, we have workaround with Firefox:
(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.
(2) In the search box above the list, type or paste dhe and pause while the list is filtered
(3) Double-click the security.ssl3.dhe_rsa_aes_128_sha preference to switch it from true to false (disable Firefox from using this cipher)
(4) Double-click the security.ssl3.dhe_rsa_aes_256_sha preference to switch it from true to false (disable Firefox from using this cipher)
One this is done, the issue will disappear.
We will keep you updated on our progress.
DevNet Sandbox Support