Almach9, have you done a strip test with the paper? If not, do so. Use the same aperture as for your successful prints and starting at, say 8 secs. Move in say 3 second intervals over 5 strips, develop the whole strip then tell us what you see.
I think you are saying that you seen the image in the developer for a few seconds( how many?) and then the paper goes completely black Have I got this right? If so, it suggests that the paper is OK but some how it has been greatly overexposed. Did you focus at say f2.8 then forget to move the aperture so that it got 18 seconds at f2.8 instead of the intended aperture of whatever you had used to get a good print?
It certainly sounds as if you failed to close down the aperture but until you do the test strip we cannot know how to help you solve your problem. Try what I have suggested, making sure you close the aperture down to the previous setting such as f8/11.
pentaxuser