Any ideas on what causes this?

I've had Steam offline for a few weeks now to avoid being autoupdated to 1.8 when it releases, since the CK hadn't yet been updated and we couldn't fix mods to work with 1.8 yet. I didn't want to have half my mods explode when my game updated to 1.8 without asking me, and the CK hadn't caught up.

Now that the CK update is here I figured it'd be safe to go back online and grab it. So I did, the CK updated itself as it should, etc. Except... now when I try to open the CK, it hangs and crashes. Immediately. I can't open the program at all, it shows me the splash screen and then hangs there for five or six minutes before the Windows error pops up letting me know it isn't responding.

What have I done wrong THIS time?

Has your game updated yet ? could be you got the ck update but not the game update yet.

Do you have any launch parameters or anything related to the SEQ stuff or something?

Try reinstalling the CK.

I'll try reinstalling. It makes me nervous to uninstall CK stuff... I'm always convinced somehow it will delete all my mods and my work in progress, even though I know damn well it won't, and I've backed up anyway. There's just something about clicking that total uninstall button that I find panicky.

I'm glad to know I haven't done anything obviously stupid, though.

Edit: Okay, reinstalling the CK didn't work, so I tried reinstalling the game update itself, and that did work. Of course, now my whole game is crashing on startup. I suspect for some reason the update was never installed correctly the first time, thus making the CK not work, and now it's updated properly so I've got to figure out which of my hundred mods isn't compatible with 1.8. (sighs)

Check your Skyrim folder for TESV.exe.
I did the same thing. I updated the CK first and did not have the game set to auto update. When the CK updated, it deleted the old (1.7) TESV.exe, but did not replace it with the newer (1.8) version. I had to update the game and verify the game files integrity. This replaced the deleted TESV.exe with the 1.8 version.

That may well be what happened, I'm not sure. I do normally turn off autoupdate, but Steam will run right over that and update it anyway if you start the game when an update is available. The only way to stop it (and still be able to play) is to keep Steam offline.

Thing is, I finally let it go online last night for the first time in awhile, and it promptly did its update of the CK, then I went into the game to grab some reference IDs I needed to work on my mod, and only then did I fire up the CK and found that it wasn't going to let me mod.

I never ticked the autoupdate box, I just assumed it had updated itself because I started the game with Steam online. Evidently for some reason it didn't, so after a reinstallation of the CK didn't work, I went poking around in Steam to figure out where it lists the current version of the game and make sure it was 1.8. Never found that, because as soon as I right-clicked Skyrim from the library and looked at the properties, a box pops up saying "Steam needs to convert your game content blah blah to a new, more efficient version," which freaked me out for a minute because I've never gotten that message before during a game update.

So, (shrug) I don't know. Obviously it never updated to 1.8 as it should have, even though I did run the game after getting the CK update, or so I thought anyway. My best guess is that since my ISP's been a bit spotty since the hurricane, it may have taken a few minutes longer than usual for Steam to check itself out and realize it needed to grab the CK update after I brought Steam online. Maybe I clicked to run the game before it had downloaded CK and thus deleted the TESV.exe. Then once I closed the game, it got its stuff together, updated the CK, but didn't update the game since I had never ticked the update box.

It works now, that's all I care about. I found the mod that was making the game CTD on startup and squashed it until it gets a 1.8 update. All is well.