The most common cause is the computer's clock being off (which can happen for any number of reasons, e.g. Windows screwing up, so not necessarily set by yourself) so I'd check your system's settings.
Good luck, Exile and have fun wherever your path takes you next. :-)
A huge download like PoE sucks but actually, a fresh install isn't that bad of an idea once in a while, anyway. Over time, the game hogs insane amounts of space and installing from scratch has shaved GBs off for me before.
Also, if you have the space, you can copy the content file to another location as a backup. That way you'll only have to download the patches if something goes wrong again.