There have been claims (e.g. Ben Davies) that the wrong filter is being used for flats: the FILTER keyword in the flat has a different filter than the one ACP uses in the file name, and showing in the log. In at least one case, it is certain that the wrong filter was actually used.
Add additional logging that can be conditionally enabled which will trace the filter usage. This should help isolate these problems.
NOTE MANY CHANGES IN ACP-1119 (linked)!
