BLANK Cloud Screen http://forums.adobe.com/message/5484303 may help
-and step by step http://forums.adobe.com/thread/1440508?tstart=0
Mac Spinning Wheel https://forums.adobe.com/message/5470608
-Similar in Windows https://forums.adobe.com/message/5853430
Thanks for the reply. I have tried renaming the OPM.db file to no effect. It was recreated but I am still left with a completely blank dialogue. I also tried renaming the entire folder and reinstalling CC. The folder was recreated but no change on the dialogue. It won't allow me to do a complete uninstall of CC because PS is installed. I don't know where to go next.
I've done some further experiments.
I uninstalled PS and was able to uninstall CC and reinstall it. Still a blank dialogue.
I re-imaged my system back to yesterday. Still blank.
Now I really am stuck.
I restored my Windows system to a known good state one week ago. CC immediately updated itself and gave the option to update PS which proceeded without incident. I still don't know what caused the problem but at least it has gone away.
Thanks for your help.