I can confirm that I am seeing this issue with all our AD users on cventer 5.5 u02 build 1623101 (latest vApp version with latest patch).
I upgraded all of our VMware hosts to 5.5 latest patches at the weekend and now most of our users cannot login to wither web or windows vsphere client.
Any update from VMware on this, nzorn?
Spaces in AD usernames were definitely working with vcenter 5.1 (last Friday) ...