Style Change in Permissions

[multiplicity!]

Hello!

I recently upgraded my 1.6.13 forum to MyBB 1.8, and everything is looking good with no issues, save one.

When I click on Edit Permissions in the permissions area of a forum (after creating the forum), it gives me this: http://i.imgur.com/K5GY6O7.png

I'm unsure of what to do about it. I've tried reloading all the caches, the pages, etc. I file verified, and the installation was done correctly as far as I'm aware (I followed the upgrade guide on here).

Any help would be greatly appreciated, as I can't change permissions effectively on those forums without the correct modal popup.

[Destroy666]

Run the ACP -> Tools & Maintenance -> File Verification and show the output here. Also provide a list of plugins.

[multiplicity!]

FILE VERIFICATION
Problem files: inc/plugins/hello.php is missing.

PLUGINS
Enhanced Account Switcher (1.5)
Activate Users from Mod CP (1.0)
Apply Thread Prefix (1.0) (changed compatibility to 1.8)
Shows Avatar on Threadlist and Forumlist (2.8) (changed compatibility to 1.8)
CKEditor (0.8.1)
Code Select All (1.0.0) (changed compatibility to 1.8)
Edit History Log (1.0)
My Awards (2.3) (changed compatibility to 1.8)
MyProfile (0.3)
NewPoints (2.0)
PM Menu Counter (1.0.1)
Private Message Log (1.0)
Quick Advanced Editor (4.0.0)
Report Private Messages (1.0)
Like System (1.4.0) (changed compatibility to 1.8)
Style Usernames (MyBB 1.6) (1.0.0)
Team Online (1.2)
User Tagging (1.0)
View All Posts by a User in Thread (1.0)
XThreads (1.63) (changed compatibility to 1.8)

INACTIVE PLUGINS
FASTyle (1.0)
PluginLibrary (12)
------------

NOTE: I have all of these plugins save the Enhanced Account Switcher on another 1.8 installation, and do not have this problem. The Enhanced Account Switcher is version 1.5, which was made compatible with MyBB 1.8.

[.m.]

looks like there is a problem with loading of ~/admin/styles/default/main.css

[multiplicity!]

How would I go about fixing it then? My internet connection is sound and I'm not sure what else would cause it?

[.m.]

if there is a change in that style sheet then file verification tool would have reported.
check if disabling the plugins or changing the web browser / clearing browser cache (again) helps

[multiplicity!]

I cleared my browser's cache through the settings of the browser three times and it finally worked.

Thank you very much! ^^