If you're able to provide a packaged workbook, you'd likely get more targeted help. More details would help too, such as: are your objects floating, tiled, in layout containers, or a mix? Specifically, which of those options is the disappearing sheet?
Some other thoughts:
- If the disappearing object is in a layout container with other objects, choosing the 'Distribute Evenly' option after selecting the container should give your disappearing object equal space in the container as the other objects in there so a bigger object can't hog all the space.
- If the disappearing object is floating, try bringing it to the front by right clicking on it in the layout tab
- Text boxes placed into layout containers can be set at a fixed height:
I'm not familiar with the internal error you mentioned or being unable to re-add a sheet to a dashboard, but hopefully some of these ideas help you out.