Due to working process collisions when several editors are trying to edit the same project configuration at the same time, WebOffice author sets the project configuration to read-only when opened. Other users trying to open the project will get the notification that the project is edited now. The read-only setting of the project may be overruled.

Read-Only notification and possibility to overrule - WebOffice author
Note: Closing ArcMap without closing the project configuration in WebOffice author or an ArcMap crash will cause that the read-only setting is persisted (you will always get the notification (see figure above) even if nobody else is working on the project). If this happens you need to remove the read-only setting project configuration XML-file on file level.