Modpack version
1.66c

Can the issue be reproduced?
No?

Logs
https://pastebin.com/EdcxKqXn

Issue
It always takes a few crashes before I finally can launch the modpack. Today it crashed 4 times, yesterday - 3. I'm scared I'll have to go through 5 crashes next time I wanna play xD.
Thank you~

You appear to be using outdated Java. This is often caused by Twitch defaulting to an older version than you have available. Follow these steps to update.

• Make sure you have the latest 64-bit Java installed
  https://www.java.com/en/download/manual.jsp Windows Offline (64-bit) is the one you need
• Go to the Twitch Minecraft settings
• Add -version:1.8+ to the Additional Arguments

Java Downloads for All Operating Systems

Java manual download page. Get the latest version of the Java Runtime Environment (JRE) for Windows, Mac, Solaris, and Linux.

Oh, apparently twitch uses the Java version (1.8.0.51 64-bit) it installs itself in [twitch app]/Install/runtime/jre-x64 directory. It can't be changed in twitch app settings (it's not clickable) but it's possible to make a modpack use any other java version in mojang's launcher. I'll try to do exactly that, thanks.

Seems to work just fine now! Thank you for help.

The last step Disco said changes Java version, I recommend you do that instead of through the launcher.