Try to delete the cache lib file maybe it is corrupted. (But make a backup of it first)
Restarting kicad might also be a good idea.
You only get a rescue lib once kicad detects there is a difference between the symbols in your libraries and the symbols in your cache lib. If that happens you will be prompted to either rescue the affected symbol(s) (meaning copy to a rescue lib) or update from the library.
The strange thing is the mention of the symbol inside of a rescue lib. Check with tools -> edit symbol library reference if every symbol points to a valid symbol.
If you transfered this project over from v4 then you might be interested in this tutorial: Converting KiCad version 4 projects to version 5 (Remap a project)