When clicking Install Now on Quickmap Data Download Manager V8.4.8622.20335, you may get this error:

Unable to install data updates

Unable to cast object of type ‘System.DBNull’ to type ‘System.String’.

This is caused by the Download Manager examining prior downloads but failing when it finds a empty data (a null file hash) from a previously-failed download.

There may be some way to clear the old download data, but the easiest way to fix this error is to upgrade to the latest version of Quickmap Data Download Manager, which contains a bug-fix to handle this scenario better.