Expression Engine won't let me change Output and Debugging Preferences

Hi guys. I’m fairly new to EE and have just set up a fresh installation of EE 2.10 on my local server using MSM to govern 2 sites. I’ve just created fresh templates for both sites’ homepages and have got everything working correctly.

First problem: while I develop the templates with CSS, I would like to remove the debugging output at the bottom of the page for clarity. However, when I try to turn it off (ie. set the radio button to ‘No’ and click submit), it simply reverts back to ‘Yes’. This goes for every option under Admin -> System Administation -> Output and Debugging Preferences, other than these two, which I CAN change:

• Generate HTTP Page Headers
• Force URL query strings

All the others it won’t let me change. There are no errors displayed on the dashboard, it just gives me the little green tick with a ‘preferences updated’ message, even though the friggin’ radio button has jumped back to its default setting.

Can anyone suggest why this might be happening please? Thank you.