You may want to force "rebinding".
"Prebinding is a process that, essentially, updates the information various applications store about where to find necessary shared libraries and other components. It can not only result in faster launch times, but also allow a non-launching application to work properly when there is a glitch in its addressing data."
1. Open the Terminal (located in Applications/Utilities)
2. Type: sudo update_prebinding -root / -force and press Return.
When prompted for a password, enter your admin password, and press Return again. The process may take a few minutes, and you may see various messages flash by. The process is complete when the Terminal returns to a normal prompt.
3. Type exit and press Return.
4. Check for the capability to launch problematic applications (in your case downloads)