Long time user of MineOS though I have just recently come back after a long hiatus. I remember having this exact issue two years back and never actually found a solution so I figured it was about time to hop on the forum.
I have been trying to put up a FTB Revelations server but I find that even after using the FTBInstall.sh button the server will not start. (Classic FTB start issue, sometimes runs for a second then stops, etc.)
I have gone to the main terminal and manually run FTBInstall.sh and it ran without errors though I know that the webui button itself is working because the minecraft 1.12.jar shows up a short while after I press the button. As usual Vanilla servers are working fine, so on and so forth.
If anybody has some ideas or input I would love to hear back as I am completely stumped on this one.