Symptoms

Workspace's Navigation.aspx loads, but it's inner Worklist.aspx frame gets error: "The server is not operational".
 

Diagnoses

After iisreset, Workspace is fine for 1-3 days. Checked the "Load User Profile" value in the IIS App Pools for K2, all were set to true. Changed Workspace's web.config file to enable custom errors in case it recurs.
 

Resolution

Explicitly added port 389 to the LDAP string in the section in the Workspace web.config located at '%InstallRoot%Program Files (x86)K2 blackpearlWorkSpaceSite', then iisreset.