Hi I found this issue post because I also had the same error---
Fatal error: Cannot redeclare class services_json in /home/content/---my path---/wp-includes/class-json.php on line 115
I now just got this working, This may be some help to others to troubleshoot their issue too.
1) I was looking at the WordPress site and it says somewhere that whilst WordPress supports 4.3 there will be a time in the future where you will need to change to php 5.x. It also says that some things that do not work under 4.3 simply work under php 5.x -so iI thought I would give it a go
There are a few cautions to changing to php5 but would only impact for a few people.
There are instructions on how to do it with various host providers.
I had to wait a few hours for the changes to filter though and once the system had changed. When I clicked to start it I no longer got the error message.
2) Once working I was then able to move and resize things.
However the boxes would not delete and the boxes would only resize once and the whole thing would not save. I was using Firefox 3.5
I noticed a message suggesting some issues with browsers so I switched to Chrome browser where I can say everything now saves and can be resized multiple times.I also get the editor now.
version of wordpress is 2.9
Hope all that helps troubleshoot this issue.