I tested the case by adding another test page and with no plugins and it still has a problem. I checked the WP Server Info and the settings seem to be ok.
We do not have such a problem in our installations that is why I have 2 possible cases in my mind which you need to test:
1) There is a problem with your installation and database, Please check the case with a completely new WordPress installation and Theme installation.
Judging the number of plugins you have already on the site it seems that you did not check the case by having a complete vanilla WordPress installation.
2) It might be a server restriction that causes the errors below:
Maybe it is the mod_security settings which prevent the builder to work.
You need to move your website to another server or your local machine to test the case.