Unless you have Auto Save set on your workbook all users will not see the updates by other users until they save the book. so this can lead to conflicts with more than one user updating a record that is not updated for all users. I don't recommend using Auto Save. You should advise your users to manually save the workbook before making a change and save the workbook after making a change, to keep the most recent updates and minimise conflicts. I like to add the 'Update File' button to the default toolbar next to the save button. This will save and refresh your workbook. You'll find this on the commands tab File items, of the Custom menus dialog. Right click a toolbar and select Customise from the menu to access it. If you have users that do not have to update only read, you may want to have them open the file 'Read Only' and then use the 'Update File' button to refresh the data.
The second message may have to do with an External link in your workbook that is change or not available. In most cases Try to avoid saving External links in your workbooks. To check for External links click the menu 'Insert->Name->Define...' If there are items in the list select them and delete or change anything showing a path to an external workbook displayed in the 'Refers To' text field.
The last message should be avoided by keeping the book saved as suggested above.