When I try to login to the admin panel it just redirects me to # then stops. So if I try to login on the page website.com/zp-core/admin.php it just takes me to website.com/zp-core/admin.php# which is a blank page.
I've reset the password several times, the email came through fine, and the website is working properly, it's just the admin panel is inaccessible.

Has anyone had this problem before or have any ideas?

Which Zenphoto version? Any errors in the server log?

It first occurred on revision 5326, and continues on 5413

Just checked server logs and can't see anything unusual.
I should note that if I enter the wrong password it works fine (it tells me it was a wrong password), the blank page only happens if I enter the right password.
It also happens in all browsers.

You might try a newer nightly then. Blank pages are most always related to PHP errors.

It's the same with the latest nightly (2010-05-14). The only way I can get into the admin panel is to click the link in the email it sends to reset the password.

I can't reproduce this or never noticed this, although there were some redirection issues before. If you get a blank page there must be an error or something somewhere. Try a clean reinstall, maybe some files are not updated correctly.

What do you mean by "clean reinstall"?
I have unzipped the nightly package over the directory and replaced all the files.
Do you mean start on a clean database and config files too?

Sometimes FTP clients and such don't replace files correctly. So I suggest to remove all files first and then reupload them (although it should not make a difference...). Just to be sure...

Does setup complain about anything?

I didn't use FTP, I just unzipped and overwrote. Setup didn't say anything except ask for the password, and when I gave it it did the same thing. The only way the setup works is by getting it to send an email to reset the password, then I click that link and it lets the setup finish.
No errors or anything.
I'll try a completely new install and see if that works.

Edit: The new installation works. So I have no idea why that would work while the old one doesn't.

I am sorry, I have no idea then what it could be.

Thanks for trying, anyway. I've fixed it by just doing a fresh install but restoring the images database table. That was mysterious!