I dont know if this would help solve the problem, but i tried running other flats from another set of observations and get to the same step. The first flat pops up with the same error as before and the other two flats run just fine. Could it be that its the first one throughout my procedures.
I too have had a problem in gprepare. I’m reducing flats at the moment from gemini north from 2013(If that is considered old or not). It would pop up with an error about mdf_suffix is not recognized and so i went into to the code and found that it is not preallocated so it just needs to be added to the list of variables. But then it gave me that the Mdf could not be found or does not exist, even though I see can see it. I found that in the if statements that pertain to mdf_suffix in the gprepare.cl at the bottom of the if statement it essentially is looking for a “.fits.fits”. i fixed it by getting rid of the //.”fits” since there is already a .fits extension in the if statements above.