The background spline maps from the pipeline and the exposure maps produced by the task eexpmap have headers which are incompatible with eregionanalyse. The simplest way of solving this problem is to replace their headers with that of the source image by using an ftool, e.g.:
farith srcimage.fits bkgimage.fits tempfile.fits "+" farith tempfile.fits srcimage.fits newbackgndfile.fits "-"
which will produce a new file, newbackgndfile.fits, with the data component from the original background file, bkgimage.fits, and the header of the source image, srcimage.fits.