I this case like said before it's most likely a name issue.. As in, when adding both wads , the images in both have similar names. Try appending a wadset name to the front of all image names..

i.e. "WD1_skyblue.tga"

Also check file name length there is a issue know there.

Final I had some similar error when import block had a second skin. This in WED was a light map but in 3dsmax was not. All blocks, had a fully black second skin that had to be deleted.

Final note... I stopped a fresh project 2 weeks ago over not being able to fully solve the "No to All' error, which is about as helpful an error code or throw message as "Somethings wrong"....