You can have multiple valid tex folders (you can find the list in your prefs) as long as the textures from one project are in a valid tex folder, then when you import a c4d file into a new one all should be good.
C4D normally asks you if you want to copy the image you are applying to a material into one of the tex folders. If you say no, it’s somewhere on your drive and will get used for that project only – it can get “lost” when you open the project again. So its a good idea to set those tex folders up and say yes when C4d asks where you want to put it.
If all of that has been done, I’ve found when an image goes missing, if I go find it for one texture, save the file as something else and then open the file back up, C4d will often find the others. Its like that missing image give c4d a hint as to where to look.