FOUND IT!
At this URL:
I tried a coule of his suggestions. The combination that worked was to reset permissions at wp-content/uploads to 777 (775 did not work) AND, go to Settings/Media in WP sidebar and in the “Store uploads in this folder” text box, put the text ** wp-content/uploads ** (NO asterisks).
And, of course, I reset the Permissions back down to more secure level.
Not only did that allow MainWP to complete the Sync process, it allowed another Theme (the deprecated Aspen, now replaced by Weaver Extreme) to Save & Download backup data.
I’ve been messing with this for over a YEAR, and this guy at tipsandtricks-hq.com had the solution.
I don’t know how common this issue is, but I’ve got it fixed now. Hope it helps someone else.
Thank You,
DSL