Mike,
Were you able to get this issue resolved? We're testing ArcGIS 10.6.1 now and are running into the same problem. Any mosaic dataset we try, no matter if it was created back in 10.3.1 or brand new in 10.6.1, no matter if it's in a file geodatabase or our enterprise geodatabase, it causes this error. It seems to be when a user tries to move the display (or otherwise click, which stops drawing) while ArcMap is still drawing the raster. Usually changing extents very quickly will cause this to pop up and disappear in rapid succession.
It's usually:
"[Layer Name]: Image: Function Read Error [Mosaic Function]".
Sometimes it's:
"[Layer Name]: Image: Function Read Error [RasterInfo Function]".
Those are the only two error messages I've seen. Refreshing or moving the map display again will cause it to go away, but an empty "ArcMap Drawing Errors" window remains open. Overviews can help the raster draw faster, which reduces the chance of the error, but it still occurs if you can click fast enough.
This is the only thread on this issue I've found, so hoping you can tell what your experience was and whether you were able to find a solution.
Thanks!
John