Permalink Custom Structure Refuses to "stay"

I’m not sure if this particular problem is a conflict with plugins used or if there’s something wrong with the core files of my wp install but I selected custom structure in this blogs permalinks…
/%year%/%monthnum%/%day%/%postname%/
that worked fine
… then I had a possibly unrelated problem with an error on line 3193 in some-file.php

in a possibly misguided effort to correct this error I attempted what I think they call a “null edit”, went into Settings->permalinks and (temporarily) changed the permalink choices to “anything other than what I had” (allegedly this forces the core files to update or something), then changed it back to the way it was…

umm, now it refuses to go back to the way it was… all of my internal links are now returning 404 not found errors
please help