DDP won't load into new montage

At end of successful DDP creation from a montage, I choose to open the DDP as new Montage. It no longer works.

The ‘Missing Files’ box appears to relink paths. It won’t find anything if I select “Find Files with Same Name” (yes, correct directory) and if I select ‘Replace Selected Files With…’ then image.dat is grayed out if the default ‘All known file types’ is selected in dropdown. If changing this to ‘Any type’ I can select It won’t recognize ‘image.dat’ as a valid file - greyed out. If changing file type from show ‘recommended’ to ‘all’, i can select the dat, but it won’t link properly. It won’t even show up in the replace missing files prompt. If I go on and save the new montage based on the newly created and missing DDP, and choose render → audio montage > replace, and select image.dat, it will hang/pinwheel and then crash. What changed?

X.15.3, WL Pro 9.5.5