The log shows the "The given path's format is not supported" error from back in October but you apparently fixed whatever that problem was and it's been working since then, except for some occasional database logon failures. That's probably a pretty straightforward problem with database permissions. You don't have any database credentials in the project, so you're using integrated security, so whatever account you're running Report Commander from probably doesn't have permission for the database.

You were using the 32-bit version of Report Commander 2.5. Now you have also installed the 64-bit version of 2.6, which is failing with "Failed to open the connection." This is probably happening because you have 32-bit database drivers and/or ODBC alias, so the 64-bit version is not going to work. You need to uninstall the 64-bit version of Report Commander 2.6, and install the 32-bit version if you want to upgrade from 2.5 to 2.6.