After trying to mount and finding anything useful, I instead went for strings and grep:

$ strings Detvarderengang.dd | grep 'Flag'

Last line is:
> Flaget er 078 067 051 123 102 105 108 121 115 116 101 109 095 105 110 116 101 116 095 112 114 111 098 108 101 109 125

Lets decode that:
$ python3 -c "print(''.join(chr(int(c)) for c in '078 067 051 123 102 105 108 121 115 116 101 109 095 105 110 116 101 116 095 112 114 111 098 108 101 109 125'.split(' ')))"
> NC3{filystem_intet_problem}