We definitely know how frustrating this is. The resolution that we came up with was to repair all of your workspaces. Luckily, we didn’t have everyone at 10.4 we still had some machines at 10.3.1 and we used those workstations to repair the data source.
**What we had to do to make this work was to create a new sde database and delete our old one. This consisted of a copy and paste, and this gave us a new connection string.**
Right Click on your database connection to sde.
Select properties.
The highlighted path is what you will need to repair all of your mxd’s.
Right click on mxd that you want to repair.
Select “set data source”
Select all
Replace all
Insert your new file path you received from your connection.
Good luck, I would try to re-set the file path first. Creating a new sde database took us sometime, and was inconvenient for our users when the maps were down. This process was recommended to us by esri support.