I am having this issue too. I ran the webui standalone with node and got the following errors:
error: Unable to retrieve profile: ftb. Please check your internet connectivity.
error: Unable to retrieve profile: ftb_third_party. Please check your internet connectivity.
info: Downloaded information for collection: Mojang Official Minecraft Jars (423 entries)
It seems like some of the profiles are being found correctly but the FTB repos have gone down. I dug through the code and found that the two links are http://ftb.cursecdn.com/FTB2/static/modpacks.xml and http://ftb.cursecdn.com/FTB2/static/thirdparty.xml which are both currently dead.
You can temporarily fix the issue until a better fix is available by commenting out lines 98 through 223 of /usr/games/minecraft/profiles.js. This will disable looking for profiles on the two dead links.