By A few days ago I wanted to edit a Master page in SharePoint Designer 2010 but I received the following error message:
“The URL ‘my custom url’ is invalid. It may refer to a nonexistent file or folder, or refer to a valid file or folder that is not in the current Web”
First, I thought that is a problem with my SharePoint Designer installation so I tried to check out the file from the browser. No luck, same error. After spending some minutes on the internet I found out that could be diferent reasons for receiving that error message. One of them could be low disk space on the MSSQL SharePoint database. In my case that was the problem, I increased the free space on that server and after that everything worked fine.
Thanks to the people who post useful things on the internet.
References: reference 1, reference 2
octavian sfetcu 