A failed save on a workspace says so in plain words

On a project workspace, the controls that change something — approving or declining an invoice, deciding on a proposal, moving a milestone, and the actions in the side rail — now say “That change was not saved.” when the save fails unexpectedly. Before, some failures showed a technical message about a server render instead.

In both cases nothing was saved, and the workspace still shows what it showed before, so you can try the action again.