I am using ESXi Host 5.5.
In the Domain field, there is only "(server)", but not my ActiveDirectory host, which I have successfully added via Configuration > Authentication Services.
What could be going wrong? How can I fix it?
Check this blog post and check your configurations:Joining vSphere Hosts to Active Directory | VMware vSphere Blog - VMware Blogs
I set up my AD using exactly this process...
What else could be an issue?
Check you ESXi computer account on AD and also your DNS records.
If computer account is not available on AD, your ESXi didn't join to the AD correctly.
Try to configure that again.
I installed some updates and restarted my Windows Server and then re-added the AD service within ESXi a fourth time, and now it works...