This error could be caused by few reasons:
1. The tab was open for a while (usually a few minutes) the user tried to save the changes.
Changes can't be saved after the tab was open for a while with no activities due to safety reasons, so when submitting an idea, please make sure to save the changes and don't leave the tab open without saving.
As long as the user saves the idea in draft mode, he/she can always go back and edit the idea at a later phase.
There is an option to be allowed to edit the idea also after submission per you system administrator decision.
2. If the idea has multiple co-authors, one of them or one of the moderators might be editing in the same time.