my better half is having this same issue as well. We have tried a few things.
1 - update java
2 - forceupdate ultimate pack.
3 - manually download twilight forest 1.15.3
4 - delete twilight forest from /ultimate/mods/
5 - copy over the manually downloaded twilight forest 1.15.3
6 - delete ftb from computer
7 - redownload ftb.exe from this site.
8- repeat steps 2-7 for the past 8 hours.
9 - come here and me post in this thread.
it worked for my computer, and another guildy. but it will not let her.
If only she had decided a day or two earlier that minecraft looked fun enough to play. Is there any way that the ultimate pack can be rolled back to the previous version of twilight forest on the ftb client? We have a couple other people that are interested in playing, but with this issue have decided not to spend a whole night trying to get it working for themselves and are waiting and playing the game that takes place in the world of azeroth..