In addition to the cyfitter file try to delete other files: workspace files, Generated_source, etc. - practically all except project file and main.c and .h. (Copy entire project folder in some backup space first).
Unfortunately, Creator is prone to ghost errors. I suspect they come from unconnected wires of zero length, which are not detectable by parsing algorithm. The fastest and ultimate solution is to create a new project and re-create schematic and copy/paste *.c and *.h files. Takes 5 min.