During the opening of Document Definition the following error may occur:
Unable to access Document Definition (<path to *.fcdot file>). This object is corrupt (<path to *.fcdot file> was not found)
The possible root of the issue may lay in not actual Document Definition, which got corrupted during the publishing of changes to the server because of connectivity to the application server or network issues.
The issue of Unable to access Document Definition may be resolved by taking from the backup project folder actual *.fcdot, which locates in "your project folder name" > Templates.
Then you just need to import this actual Document Definition *.fcdot using the Import option: