This sometimes happens if you are using Notes with a database that has an R5 on disk structure.
Right click on the background of the workspace, and choose "Workspace Properties".
Click on the white i on the blue background.
Increase the value in the box by 5.
Click on "Compact".
Try opening the database, if it still gives an error then do the following:
Make a doclink of the problem database, by selecting it then "Edit->Copy as link->Database Link" and paste it into a new Outlook/Notes memo.
Remove the problem database from the workspace, by right clicking on the icon and selecting "Remove from workspace".
Re-add the problem database by clicking on the doclink.
Delete the Outlook/Notes memo.