I am also experiencing the same issues. I am on Asia server, running windows 7 (I know, don't judge me) 1.15.0 worked fine first go. Login screen reached but then just hangs indefinitely until I force end to processes in task manager. Same issue on laptop running windows 8. Downloaded solo's modpack and it worked fine (on laptop). Then reinstalled Aslain's modpack and it seems to be working on the laptop.
I am going to try the same process on desktop and will post the results if this works.