Thank you very much for the valuable feedback on the migration module, it is much appreciated! I'm sorry to hear you are experiencing the below issues after migrating your site, let's try to solve them out:
1. Judging by the provided screenshots, it looks like there might be a ContentPlaceHolder declared in the head section of the masterpage you were using as a template for the migrated pages. If during the migration som controls did not have a valid placeholder, they would have been moved to the first available placeholder on the page (in this case the ContetnPlaceHolder in the head, which is not a valid editable area). Commenting out that particular ContentPlaceholder from the masterpage should help fixing that behavior.
2. Once you have done this, and if any problems still persist, can you please inspect with developer's tools (e.g. Firebug) whether any errors get registered while editing/publishing the page.
3. Concerning the invalid server tag, indeed we do not have telerik:JsFileLinkcontrol anymore in Sitefinity 4.x however, you can use the ResourceLinks control to implement that functionality, or share with us the exact use case scenario, and we'll try to give you a more specific advice.
Looking forwards to your reply.
All the best,
the Telerik team
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