You probably will see similar issue below if you upgrade vCenter Server from 5.x to 5.5.
vSphere Client show following error when login to vCenter Server by domain account.
The vSphere Client could not connect to vCenter server. The server vCenter server took too long to respond. (The command has timed out as the remote server is taking too long to respond.)
You are not able to login to vCenter Operation Manager by domain account with error message below.
Incorrect User name/Password
This issue looks like related to how vCenter Server fetch domain account information in Active Directory. vSphere Client and vCOPs may get timeout during fetching if the account in large number of domain groups. You have to increase timeout to avoid this problem by following KB articles Unable to log in to the vSphere UI after registering a new vCenter Server to vCenter Operations Manager (2045029) and vSphere Client times out before authentication to ESXi completes (2072539).
Please notice that, there is an option to set vSphere Client timeout value on vCenter Server (Administration -> vCenter Server Settings -> Timeout Settings -> Normal Operations), it apparently same setting described in KB 2072539 but server side. Somehow it doesn’t work for me, I guess that’s because I didn’t restart vCenter Server services after changing it. Please give a try.
Chinese version:
如果你的vCenter Server从5.x升级到5.5了，有可能会看到下面这个错误。
在用域帐号登录vSphere客户端时可能看到如下提示：
The vSphere Client could not connect to vCenter server. The server vCenter server took too long to respond. (The command has timed out as the remote server is taking too long to respond.)
在 用域帐号登录vCenter Operation Manager时可能看到如下错误：
Incorrect User name/Password
这个问题可能和vCenter Server抓去域帐号信息的机制有关系。如果你的域帐号属于很多域组，在vSphere客户端和vCenter Operation Manager时就会出现这个超时问题。可以通过增加超时时间来解决这个问题。具体请参考知识库 Unable to log in to the vSphere UI after registering a new vCenter Server to vCenter Operations Manager (2045029) 和 vSphere Client times out before authentication to ESXi completes (2072539)
在vSphere客户端也有一个可以修改vCenter Server延时值的选项（管理 – vCenter服务器设置 – 延时设置 – 一般操作），但是我试了一下好像不起作用，也许是因为需要重启vCenter Server吧。