I was getting an error when saving a Data Source Content type with a large number of variables. Date invalid fromat.
In syslog there was a message: apache2: PHP Warning: Unknown: Input variables exceeded 1000. To increase the limit change max_input_vars in php.ini.
After seaarching I found: https://2bits.com/drupal/drupal-not-saving-admin-pages-large-number-inpu...
Configuring PHP to accept more input variables
The default value for input fields for PHP is 1000. While this is sufficient for normal sites, it is not so for sites that overuse (misuse/abuse?) Drupal features.
I increased it to 2000 and then was able to save the file.