LRGB sizing error

Ok, it's just happened again on fresh, good data.

I imaged in LRGB on my ASI1600 with the RASA.  A set of Optolong LRGB filters.  I stacked them separately, all with their individual flats/dark flats, and a single darks file for all of them.  All 4 of the stacks looked good by themselves as B&W images.

So I then took the saved stacks to tools, combine LRGB.  There I added the 4 stacks.  Is any of that wrong or not advised?  Because I am doing something FUNDAMENTALLY wrong to continue to get this error.  All of my exposures are 1x1 and so are the flats and dark flats and darks.  All of the exposures are of the same time, at the same gain, at the same camera temp.  The only thing that varied between channels was the exposure time of the flats/flat darks, in order to make them the correct brightness (middle of the histogram).  For integration, I checked the 3 multi-session boxes each time.  Is that wrong?

I get the attached error message.  At the moment, I am not getting why APP says these files are of incompatible sizes.  The fact that it has happened twice shows me I am making the same mistake, but it is not in data acquisition.  Those stacks look fine!  What can cause images taken and stacked exactly the same way to report that they are incompatible??  I think, once I see my mistake, I will always know how to load these in the future.