OK, let me know if you like more information or access to a portal and see it live:)

I have reproduced it in 5 portals so I have no doubts its a bug.

Hi Jan,

on a new install the Forum module and it fails, the artifacts of the Database are not complete installed. So you have to delete manualy the all the Forum artifacts like spocs, functions views and tables from the dotnetnuke database, befor you can retry a new install.  On some testing with early DNN 5.x beta I saw this issue, but could not see the issue on the forum sql scripts.  Please double check your permissions to the database and to all DNN files and folders. 

Before retry to install new the forum module you have to check the database after uninstall the breaked forum modul, and delete all still existing forum artifacts from the forum in your database.

Jan,

the problem is not the check IF a database artifact EXISTS.  The problem is that on install the SQL is not complete running, so you have brocken database artifacts, where i.e same Forign-Keys(FK), DF, Constains not complete setting up, and couldn't so not delete.  On running uninstall the and a failure comes up the SQL uninstall stops.  Artifacts so still in the database and couldn't delete. 

The uninstall SqlDataProvider must not check if a artifakt hat previously was installed doing exists, it have to do uninstall all all of the previously installed artifacts.