Hi Jordan,
Did the user run a merge that wasn't checked-in before trying to run the Incoming Changes?
If the answer is yes, he needs to run a new merge from the last changeset in the branch (or the branch itself) to retrieve the latest changes in the branch before checking-in his local changes.
The Incoming Changes is not able to run if there is a pending merge to checkin in the workspace.
You can check the pending merge links in the Pending Changes view: https://www.plasticscm.com/documentation/gui/plastic-scm-version-control-gui-guide#MergeinthePendingchangesView
If the answer is no, I guess he run an Incoming Changes operation that threw some error.
The problem is that the Incoming Changes operation failed in the middle (because internally it runs a merge in a first stage).
Could you send the client log files to support@codicesoftware.com. You can fin the log files (plastic.debug.log.txt) located in the plastic user folder
C:\Users\xxxx\AppData\Local\plastic4\logs\plastic.relevant.log.txt
To continue using the workspace, he can try two different options:
manually backup the local workspace changes, undo all the workspace changes, reapply them manually in the workspace and retry the operation.
shelve (and undo) the workspace changes, update your workspace to the last changeset in the branch, apply the shelve in the workspace and checkin your changes.
I hope it helps!
Rubén.