Attached file is missing
During sharing, a message like: "Attached file is missing - D:\Qg4\MyUser\C505306F534B44ECB1AB7038B6I1E2\Job\7\1\{0c707283-f87d-4455-b46f-05c97f71f8db}\NP19_0_WS.QuadriModel.WS Default.cf.xml" might show up, cancelling the share.
This is mostly from some WS task that has not cleaned up properly after deleting things. The problem might be fixed as follows:
Find which WS task has the problem. Be sure the "Properties" window is showing, click the cogwheel in the upper, right part of this and check to show "Internal properties". Then select the WS tasks in the project one by one until you find one with a GUID that matches the GUID inside the brackets in the file path. In the above example, that would be the italic, underlined part of the file path.
Reserve that WS task
Go to CAD, to the WS ribbon and select this WS task. Then open the construction dialog and run "Tools", "Clean up...".
This process should recreate the missing files and also delete any mess left behind by this task.