How to disable magic accent character keys

Have a look in System, Preferences, Keyboard Shortcuts, and see if "/" has an action associated with it. If it has, you can edit it.
Normally, to get special characters, you need to set up a compose key, so you can also check in Preferences, Keyboard, Layouts, Keyboard Layout Options, Compose key position to see if "/" has been given a special action.
Also check if Compiz is interfering here - it also uses its own compose keys.

Thereś no Ṕrefernces' under system on my machine -- the closest is settings -> settings manager -> keyboard, which has a shortcuts tab. Thereś no association listed for any of the quote chars -- all the shortcuts are Alt+something or Control+Alt+something

Then the only other thing I can suggest is to check your Language Support settings (System, Administration, Language Support) and see if your default language is correct and that it has a mark in the box next to it on the list.
I have "Enable support to enter complex characters" ticked - you could try it without.

It wasn´t ticked, but I tried it both ways anyways, but it seem to have no effect. (Default language is English(US))

I do not know a definitive answer to this problem.

If faced with the same situation I would re-install some packages from Synaptic starting with the fonts on the assumption that a package has become corrupted during the update.

Ok, after lots of searching and fiddling, I figured out a way to fix this, thou not why it happens in the first place. What I did was to create a ~/.xmodmaprc file containing:

keycode 49 = 0x60 0x7e
keycode 48 = 0x27 0x22

...and put a line in my ~/.xsessionrc file to use it:

xmodmap .xmodmaprc

That hardcodes the two misbehaving keys to generate the correct characters, but is specific to my keyboard layout; someone with a different keyboard would probably need something slightly different.