404 Errors when making updates or changes

Hello everyone!

For a while I’ve been having issues with just one of the websites I manage. When I try to update some plugins, I get redirected to my website’s 404 error page. I noticed this happened when I tried to update all plugins at once, so now I do one at a time. I was able to update all plugins except for “Velvet Blues Update URLs” this time around.

I also am unable to delete the standard WordPress themes, as I get redirected to the 404 error page. I’ve tried to disable my plugins one at a time to pinpoint the problem, but nothing came about from that. I also get the 404 error message on occasion when trying to post a new page.

I’m nervous to update my WordPress version to 3.6.1 because I don’t want the entire website to crash due to the 404 error. ANy insight on what could be causing the problem?

Below I’ve listed the theme and plugins I’m using, I hope this helps:

Theme: Cruz by Saurabh Sharma

Plugins (there are a lot, all are up to date except for Velvet Blues):
All in one Favicon
Contact Form 7
Easy FancyBox
Fast Secure Contact Form
FlexyTalk – Free Live Chat Widget
GoogleTranslate
LessThanWeb – Testimonials
NextGEN Gallery by Photocrati
NextGEN Scroll Gallery
Private Pages
Quick Cache
Quote Rotator
Quotes Collection
Really Simple CAPTCHA
Twitter Widget Pro
Ultimate Category Excluder
Use Google Libraries
Velvet Blues Update URLs
WordPress SEO
WP Google Fonts