406 Not Acceptable in Section Editor
I've not experienced a 406 before, but in doing a little research it appears to often be caused by a Mod_security rule being tripped. I sometimes get a 403 error with Mod_security when saving in CMS Builder and it's always content related.
If you have direct access to the server and can either permanently or temporarily disable that specific rule server-wide just long enough to save the record. Alternately, if you don't have server access, but can access the hosting account for that specific domain, you may have access to temporarily toggle off the Mod_security service prior to saving the record.
Let us know how it goes.
Yea, it's probably a server issue related to mod_security or a related feature.
Try renaming the partners field to something else, and try saving different content. See if you can minimize the content that it takes to trigger the 406 error. Often we do this and find we can trigger the error just by adding something like ?partners=http://domain.com to ANY url on a site. Then it makes it really easy to forward that example to the web host when asking them to fix it.
Also, you can check if any deteected security modules are running under: Admin > General > Security Modules (at the bottom)
Hope that helps!