The error you are facing is a timeout error. In order to fix this issue, there is a need to update the [User session timeout] system setting and to make sure to remove the Caching option, allow to modify it by internal users, and increase the Default value to 420. After this, the user who performs the source code generation should relogin.
The procedures should be initiated in a browser different from Google Chrome (for example Mozilla) and as an extra option - in Incognito mode. This way the issue should not occur again.
Let me also mention, that the current issue has been already resolved in 7.18.4 version of the main application, so in case the issue persists despite all the provided above recommendations, please proceed with updating the instance.