Contacted SiteGround and studied NextGen forums. The problem seems to be a bug in the PHP software version 7.0.26 used by SiteGround . PHP is the computer language used by WordPress to communicate with the SiteGround server. Apparently this bug is fixed, but not sure when the fixed PHP version will be available at SiteGround. NextGen suggests reverting back to a lower numbered version of PHP which I can do myself. But I suspect eventually we will want to come back to the SiteGround default PHP version so I am hesitant to revert to an older version of PHP. The NextGen author is saying in the forums they are working on a better solution than reverting to an older version of PHP. So I think we should wait for a new NextGen version. If a new version does not come out in a couple days, I will reconsider our options such as reverting to PHP version 5.6.