If it's about the extra jar file named "ImmersiveEngineering-[version]-core": This is intentional. It's Forge convention to make Coremods into a seperate thing. This jar is not an unauthorized download, it is shipped with IE and automatically unpacked by Forge.
Otherwise, please use the searchfuntion to make sure an issue hasn't been reported already. This avoids duplicates. Also, please give this issue a descriptive name that contains the game version in the title, eg.: "[1.12] Capacitor Interaction Crash"
Describe what happens, how it affects your game/gameplay and provide screenshots where possible.
If your Minecraft instance has crashed, a file will have been generated in the folder crash-reports of your minecraft folder. To understand what has happened, we need to know the content of that file. But please don't just put it directly in your report (that makes it hard to read), upload it to a site like pastebin or gist and put a link in the actuall bug report.
IE and Forge versions are mandatory. Note that saying "latest" is not valid. Always give the proper version number Other mods should be listed (with their versions) in a pastebin or gist upload. The easiest way to get these lists is a crashlog, you may force one with F3+C. If your game crashed (see above) you may omit the modlist, but IE and Forge version would still be nice.
Also, please try testing an issue in a minimal instance, with just IE, Forge and maybe JEI. That makes elminating compat issues easier.
Please, for the love of all that is voxelbased, read the contribution guidelines. They are linked above in the yellow box. When reporting issues, these are crucial, and the team withholds the right to close an issue for the plain stupidity of not reading and adhering to them.
Thank you for reporting issues, without people reporting them, they wouldn't get fixed. You're helping contribute to this mod and that is much appreciated <3