ModernFix Sided Setup Crash

[pawjwp]

Bug Description

I have been experience a crash where a number of components from different mods state they were already registered. The basic error for each component is copied below and the full crash report and stacktrace are in the logs at the bottom:

java.lang.IllegalArgumentException: NetworkDirection Channel {mob_grinding_utils:network} already registered
java.lang.IllegalArgumentException: NetworkDirection Channel {corpse:default} already registered
java.lang.IllegalArgumentException: Duplicate criterion id farmersdelight:use_cutting_board
java.lang.IllegalArgumentException: NetworkDirection Channel {curios:main} already registered
java.lang.IllegalArgumentException: NetworkDirection Channel {lostcities:lostcities} already registered
java.lang.IllegalArgumentException: NetworkDirection Channel {simplyjetpacks:simplyjetpacks} already registered
java.lang.IllegalArgumentException: handler pneumaticcraft:core_components is already registered!
java.lang.IllegalArgumentException: NetworkDirection Channel {forge:split} already registered
java.lang.IllegalArgumentException: A command suggestion provider is already registered with the name mantle:valid_tags
java.lang.IllegalStateException: Duplicate registration for inspirations:collector
java.lang.IllegalArgumentException: NetworkDirection Channel {diet:main} already registered
java.lang.IllegalStateException: Duplicate registration for bhc:heart_amulet_container
java.lang.IllegalArgumentException: NetworkDirection Channel {polymorph:main} already registered
java.lang.IllegalStateException: Handler for gadget_building already registered
java.lang.IllegalStateException: Duplicate recipe ingredient serializer: functionalstorage:woodless
java.lang.IllegalStateException: Duplicate registration for type ad_astra:lunarian

This crash doesn't specifically mention ModernFix but only happens when ModernFix is installed along with a set of ~16-100 other mods. It is by far the most inconsistent error I have ever encountered. It seems to only happen half of the time, sometimes with long stretches of failures or successes in a row. (For a while I was able to successfully launch by deleting and regenerating all the config files every launch, no idea if that was just a coincidence or not).

I cannot find any pattern to the successes or failures so it's difficult to try the binary search mod removal method to determine what other mods could be contributing to this. I eventually narrowed it down to ModernFix with many hours of relaunching the game with different mod combinations. I have confirmed the crash only occurs when ModernFix is installed (after many dozens of launches with and without the mod to ensure that it isn't a fluke).

In my original attempts to identify the error, I tried arbitrarily removing mods that were not specifically mentioned in the list above to see if anything else was contributing. When I got down to around ~100 mods, the crashes started reporting this error instead (full crash report and stacktrace in the logs at the bottom):

java.util.ConcurrentModificationException: null

The ModernFix version I did most of my testing on was 5.24.4-mc1.20.1 but I tested a few times on 5.25.1+mc1.20.1 as well (newest version at the time of posting) and still encountered the issue(s).

Reproduction Steps

I genuinely have no idea how to reproduce this because of how inconsistent it is. I am just hoping the logs are enough figure out what the problem could be.

The closest I can provide to reproduction steps is to download all of the mods listed in the crash report and try to run the game, but even that might not work. I tried testing on two additional computers with the exact same set of files that caused the crash as the first. On the first computer, I got 8 crashes in a row. The second computer only had the crash 2/8 times, and the third did not have any crashes after launching around 25 times.

On my main device I have successfully ran the game with all ~160 mods half of the time and the other half it crashes. The smallest mod list I have been able to reproduce the crash with is ~100 (see the second log/crash report below), however that seems to show a different set of errors, as mentioned.

I'd be happy to provide more information to help test if the logs below aren't enough to draw any conclusions, but I have no idea how exactly to reproduce this consistently.

Log File

I am including two different logs and crash reports here with the two different errors. The first is with all ~160 mods used in this modpack and the second is a smaller list from when I was trying to remove mods for testing purposes

The first crash shows that a number of components are already registered when certain mods try to register them again.

The second crash gives a ConcurrentModificationException. The suspected mod is listed as MoonlightLib, but the crash still occurs without it; when it is removed, no suspected mod is listed at all.

Both of the crashes only occur when ModernFix is installed.

---

Desolate Planet crashed. The logs have been uploaded to gnomebot.dev:

latest.log | debug.log head / tail (~26k lines) | crash-2025-11-18_18.27.24-fml.txt | KubeJS: client.log / startup.log | crash_assistant_app.log | modlist.txt

Mod list changes beyond the latest successful launch:

Added mods:
modernfix-forge-5.24.4+mc1.20.1.jar
1 added, 59 removed, 0 updated.

---

Desolate Planet crashed. The logs have been uploaded to gnomebot.dev:

latest.log | debug.log | crash-2025-11-19_11.06.56-client.txt | crash_assistant_app.log | modlist.txt

Mod list changes beyond the latest successful launch:

1 added, 59 removed, 0 updated.

[embeddedt]

This type of issue will be nearly impossible to debug with more than 3-5 mods. But one thing I suggest trying is removing the mod "Cold Sweat" and seeing if the crashes still happen; it does something very suspicious with the event bus that I suspect could cause problems with other mods.