i have seen those problems if the selection list from the inc job is "too much" different from the full job. then it does not recognize the full backup object. it is strange because the inc job before was well. but those things can happen
anyway, what you could do is delete the inc job. then make a copy of the full job (so that you have the same selection list) and change all your needs (protection, schedule and so one) to what you need.
not supported but also working:instead of deleting the whole job you can also edit the two configuration files from those jobs. copy just the selection from the object which changes to full from the full backup file and copy it to the inc backup file. make sure that you not enter space or tabstops and so one into the file...
in generall the best way is to create the full job, do the selection you want. then copy it for the incx job.
hope this helps