Little_Evenstar,
Have you tried LibreOffice Draw, Xara, sK1 or other free pdf tools such as FoxIt, SumatraPDF, Nitro Reader, PDF-XChange Lite to "resave" the pdf file? Sometimes pdf files carry hidden cruft that needs to be removed and using another pdf application to resave the file will remove this and allow the pdf file to be placed. It might also be a 'newer' version of pdf which is 1.7 + Adobe extensions that Scribus cannot interpret. I have ran into this problem with pdf files created from the newer Adobe Creative Cloud applications. The newest pdf files from Adobe will not work in or be placed into Adobe Creative Suite 6 and older version... could be the same thing here. Try to get the pdf version back to pdf v1.5 for placement. Hope this makes sense.
Cheers!
mnawij