You can try with Replace item in RME (CTRL+Shift+F) to replace the red tile with different/existing one or even replacing it before downgrading to 8.0 with a tile that exist on both clients.
You can also try the Cleanup option in RME to see how it will look like after a map cleanup.
use cleanup option to removed red tiles and redraw everything manually or use something like this: OpenTibia - Convert any map to 7.6 (https://otland.net/threads/convert-any-map-to-7-6.259410/)Anyone please?