Hopefully, if you are running those old versions, you have applied manual security patches. Regardless, it could be anything from the version of MySQL being used to exploits implanted by hackers who have gained access to your system through unpatched attack vectors. Without a lot more information, there’s not a lot we can do to help other than recommend upgrading to the latest version of MODx and start from there.
I’m using Modx 0.9.5 and get the following error when I try to add a new document in the admin:
An error occurred while attempting to save the new document: Duplicate entry ’0’ for key 1
Also, when I login I get a blank windows explorer pop-up with a yellow warning explation mark. If I click the refresh button in the browser it takes me to the admin area as a logged in user. If I click the OK button, I am prompted to login again.