SSRS Publishing error: A cobalt error was thrown

Have a SharePoint 2013, SQL 2012 SSRS site.  Integrated mode.  Report templates set up on one site, published to another (in case this matters). Publishing has been fine (once every 5 minutes, for a couple dozen reports).  No sign of any performance problems.  Now one report sends a blank page in place of a report, with the 'Last Results' on the Subscription page recording "Failure writing file <url>:mhtml. A cobalt error was thrown.

If I delete the report, the file will publish again, at least the first time or two, then will fail again.  Any ideas?

Thanks,

David

David Ridenour