Unable to Edit existing blocks or add new ones

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For some reason I can no longer make any changes to the site I'm building - when I hit 'Save' I get an error message of the form:

"{"aID":"142","arHandle":"Main","cID":"177","error":false,"bID":"834"}#next_pages_container { width: 5px; hight: 5px; position: absolute; top: -100px; left: -100px; z-index: 2147483647 !important; }"

Seems to make no difference what type of block I try tp add or edit.

This is the first error condition I've encountered a few weeks in to using C5, and naturally it's chosen to manifest itself when site's almost ready for internal review. :-|

I'd be very grateful for any suggestions as to what my be causing this.

UPDATE: Turns out that cancelling the changes and saving the page results in the changes actually being implemented, so not stopping development, just somewhat unnerving to repeatedly see and ignore an error message.

FURTHERUPDATE: Now I can't do anything becuase I get "Fatal error: Call to a member function getController() on a non-object in /home/sites/bjss.biz/public_html/updates/concrete5.6.1.2_updater/concrete/blocks/search/templates/tag_cloud/view.php on line 7. Eek!

Thanks.

Environment Info:

# concrete5 Version
5.6.1.2

# concrete5 Packages
CoolInput Search (1.0), Cube Testimonials (1.0.4), Featured Content Slider (1.0.5), Hutman News (3.0.1.2), Image Slider (1.1), Luminosity Theme (1.5.2), Power Slider Lite (1.1.1), Related Content Panel (1.1), Slate Theme (1.5.1).

# concrete5 Overrides
None

# Server Software
Apache/2.4.4 (Unix)

# Server API
cgi-fcgi

# PHP Version
5.2.17