404/Page not found error on WordPress admin – caused by plugins??

I have recently encountered a problem with accessing my WordPress admin. I am able to log in, but once logged in, rather than opening to my dashboard, I am redirected to “Page not found.”

It was working fine on Friday (Aug14). I went to “Upgrade my network” at some point on Aug 14, however I was able to work on my site after the upgrade. Since then, I have restored a backup from Aug12. Still doesn’t work.

After quite a bit of troubleshooting, I have come to the conclusion that it must be my plugins? I have gone into my wp-content folder and renamed the Plugins folder to PluginsXX so WordPress cannot find my plugins. Once I do this, I am able to access admin no problem. From here, I have attempted to identify which plugin is causing the problem. I have duplicated the PluginsXX folder, changed the name back to Plugins and began deleting my plugins 1 by 1 (refreshing my site after each one)… Only once I have deleted ALL of the plugins am I able to access admin on my site (I even tried deleting them in reverse order)….

ALSO: When all plugins are hidden (in “PluginsXX” folder) and I have access to admin – everything appears to function normally EXCEPT as soon as I click on “Add New Plugin” I am redirected to “Page not found.”

At this point, I am at a loss… Please help!