Hi,

I was preparing a new slave site with own images directory on a subdomain. Now that I have finished the site, I wanted to direct this site to the maindomain. I have entered the domain name in the List of domain names (Site details). After saving the error "Error writing configuration file" appears!?

Best regards - ikho

The "configuration.php" is the file that contain the joomla "Global Configuration".
When updating the "Global Configuration" from the back-end of the slave site, Joomla set the file in read only.

When you modify a slave site definition from JMS, the "configuration.php" is rebuild based on the parameter that you have entered in JMS.
As you probably have updated the "configuration.php" file from the back-end of the slave site, the file is read-only and can not be written by JMS.
JMS does not change the permission to force writing the file because you perhaps have changed fields that you didn't reflected in JMS.

If you are want that JMS is able to write the "configuration.php" file, you need to change its permission manually.