i post two file
I have opened your .brd & get the same issue as you, can't do anything with the .pro file.
If I create the board assembly & then open the board part & examine the "Board_Outline" sketch there is an error that would stop the board extruding in one of the corners. I assume there is a problem with the board profile in Altium or the export process. Take a look closely in Altium around the corners, one corner has a very small chamfer which causes no issue but not sure if it should be there & the other has a multitude of points & an extended single line. Once I corrected this the board would extrude OK but of course the through holes won't extrude cut automatically unless you can correct at source.
Not sure if it's relavent but I don't get an .brd file when I export to IDF, mine are .idb files for board & .idl files for library components....we use Easy-PC, not Altium here.
Retrieving data ...