Wow, great job taking your site down to the basics to eliminate the idea that the issue could be due to a 3rd party plugin and/or theme. Bravo! Also, making sure the issue doesn’t have to do with your current admin, trying different browsers, etcetera, again, bravo!
All that said, hmm, I think I am as lost as you are. 😉 But I wanted to let you know I am Googling around right now and trying to help.
Oh, have you made a list of any kind of difference between the two sites? That is, is there anything different between the site that works and the site that doesn’t work? This info would be very helpful.
One last thing for now in regards to accessing the two sites, are you accessing them with the same computer/browser???
So my Googling lead me to a now closed WordPress bug report:
https://core.trac.wordpress.org/ticket/14530
Might I suggest you put together a Steps to reproduce list of exactly how to reproduce the issue (i.e., include as many details as possible) then take that list and paste it as a comment on the bug report.
Thread Starter
amisc
(@amisc)
Yes, I read over the link above ticket/14530 and did not find anything helpful to me.
I am using the same computer and browser to access both websites one works fine the other doesn’t. The sites have different content but otherwise not really different. Some of the plugins are different but since I disabled all plugins and still have the error on the site I assume it is not a plugin problem.
Thread Starter
amisc
(@amisc)
The steps to reproduce work only on the one site and not the other. It is simply going to theme customize and changing a font color and then trying to save that gives the error.
Thread Starter
amisc
(@amisc)
Ok, so I finally tried updating/re-installing my current version and the error went away. I was going to try the manual install but tried the re-install now option first and my error is gone!!! Not sure why I waited so long to do that! Must have been corruption that was resolved with the re-install.
