Hi Guys, I had this problem myself and I did a bit of troubleshooting.
In the console, it shows the link for the mindcrack pack source, so I thought I guess I'll download it directly, and here's how to get it working.
1. Go to your FTB Directory and delete it.
2. Go to %AppData% and find ftblauncher. Delete that folder.
3. Go to the feed the beast website and download the launcher again.
4. If using the mindcrack pack, go to this link http://www.creeperrepo.net/direct/FTB2/f80d47749825a232214cfe56556130ff/modpacks^MindCrack^8_1_1^MindCrack_Client.zip
(I don't know other packs, but if you go into the console when you get the "Error downloading modpack!!!" there should be two links, click the top one.)
5. Once downloaded, go into the zip.
6. If you have installed the launcher again, in %AppData% there should be another ftblauncher folder, go into that folder.
7. Go to modpacks, to the modpack you use, then take the two folders that are in the ZIP you downloaded, and put them in the modpack folder.
8. Boot the launcher, click the modpack you want to use.
9. It should say "Downloading Modpack..." Cancel this.
10. Wait, and it should say downloading minecraft.jar or something like that.
11. Let it run, and you're ready to go!
I hope this helped anyone with the problem.
-svndi
In the console, it shows the link for the mindcrack pack source, so I thought I guess I'll download it directly, and here's how to get it working.
1. Go to your FTB Directory and delete it.
2. Go to %AppData% and find ftblauncher. Delete that folder.
3. Go to the feed the beast website and download the launcher again.
4. If using the mindcrack pack, go to this link http://www.creeperrepo.net/direct/FTB2/f80d47749825a232214cfe56556130ff/modpacks^MindCrack^8_1_1^MindCrack_Client.zip
(I don't know other packs, but if you go into the console when you get the "Error downloading modpack!!!" there should be two links, click the top one.)
5. Once downloaded, go into the zip.
6. If you have installed the launcher again, in %AppData% there should be another ftblauncher folder, go into that folder.
7. Go to modpacks, to the modpack you use, then take the two folders that are in the ZIP you downloaded, and put them in the modpack folder.
8. Boot the launcher, click the modpack you want to use.
9. It should say "Downloading Modpack..." Cancel this.
10. Wait, and it should say downloading minecraft.jar or something like that.
11. Let it run, and you're ready to go!
I hope this helped anyone with the problem.
-svndi