Documenting and diagramming code flow within a VBA Excel project

I have been using VBA for some years and have many, increasingly complex applications. I am at a point now where I am spending large amounts of time trying to debug my applications. Procedures which have worked successfully in the past are not working now. I suspect it is to do with the use of events and turning events on and off. But I also use a lot of code tracking procedures and I protect my worksheets. I think that my solution requires me to choose a method of showing my programme flow more visually and to use a diagramming methodology for doing this. I wonder whether I should invest my time in converting my applications to object-based methodologies using classes, and move to UML diagramming. I would greatly value advice on this matter. I attach an application that is confounding me at present. Many thanks for any assistance you can provide.
2017-03-24_SEBIS-Cash-Book_Developm.xlsm