Dell Command Update 3.1.2 hangs with "Operation in progress".

DCU has worked very well on this system for several years, and the current version (3.1.2) has worked well for some months.  Suddenly it has started hanging with the mesages

• "Operation in progress"
• "Warning: An operation is currently in progress. Please wait until it completes."
• "Operation is 10 % completed..."

I have tried disabling the antivirus software, running DCU with admin privilegies and running the system in safe mode (then DCU did not start at all).  I have also tried reinstalling and donwgrading to previous 3.x versions.  Nothing helps.  Any suggestions on how to fix this?