Well I did the test and I still get the red errors, below are are the images of the error, the forge and list of the mods I use.
https://gyazo.com/7411d03f004bb4661d91225b1b8368b7
If you can't see the error message its, indexOutOfRangerException: Index was outside the bounds of the array
AH HA! well, not so much, but I was able to load all your mods and replicate your indexOutOfRangerException after a bit of fighting with it, and saw something really weird that might explain the "negative amounts" but I could mot replicate the negative amounts. But what I see is enough I think, to form a hypothesis...or a wild ass guess anyways... and thanks for providing screenshots!
Short answer: I will remove/decom/deactivate the 1Million forge mod for a19, and no one should use it or attempt to make another 
. There are oddities beyond my understanding and I do not want to have a mod capable of causing issues like this on its own. Until someone can explain what is happening, its going to have to be put aside.
If you're going to remove the mod from an existing game (probably a good idea, unless you can start the game over without the mod), I *highly suggest* doing these FIRST:
1. Make a backup of your game files.
2. Any forge that has (or had?) > 30,000 of resources in a slot, dump the forge resources to get it under 30,000
3. Maybe even safer? If the forge block is "bugged" (hard to say if its isolated to the forge block), it may be even safer to completely dump the contents of all resources in the forge, then *destroy* the forge. you could likely just craft a lot of forged iron, etc, then dump whatever extra is in there.
4. Exit the game, all the way to Steam.
5. Then remove the
MeanCloud__ForgesHold1M mod
I have not tested trying to remove this mod, just this is what I would try personally!
Longer answer/thoughts (should anyone want my thoughts):
1. The mod is a 1 line mod, just changing the "stack size" of the base "unit_iron" from 30,000 to 1,000,000. all other forge unit materials extend from this. Its so simple, there's really nothing to "fix" in it even if I could.
2. Because #1, the issue likely resides inside the game not liking a number higher than 30,000
3. Delving deeper, for whatever reason, in vanilla TFP has 2 forge slots, and the stack size of the forge "units" is hardcoded to 30,000.
30,000 is not a special number, but ....its awfully close (under it specifically) to the positive standard or "short" signed integer value of 32,767.
There are 2 slots... which turns into 60,000...and the unsigned integer value closest to that is of 65,535....
so maybe TFP make the hardcoded value 30,000 because its a nice round value AND
a. 2x of this (because of 2 forge slots) fits under (inside) an unsigned integer value of 65,535.
b. Maybe not "a". Maybe because each individual slot fits under 32,767.
4. Its impossible to tell or imagine at this point... but anyways...
a. You were able to get a negative value in your forge.
b. I was testing and added > 32,767 of clay to my forge...and it works fine (Appears to work fine). I logged out, shut down the game. Started game back up and ... my forge had some weird value less than what I put in (like 23,343....but it was not negative!). But this does show that that game is capable of "removing" forge contents after a restart....maybe something was trying to store a larger number and some bits got cut off/rounded in the process...
I then added > 65,535 clay to my forge ( like ~100k), and repeated the shutdown/restart process...I got the indexOutOfRangerException AND my forge had completely disappeared!
So: My
*complete and utter guess* is that inside the game somewhere, TFP has allocated an integer (signed or unsigned) to store a forge slot value or to do some math with it. Normally its not an issue because they hardcoded the value to be allow no more than 30,000. Then, somewhere else, either a different data type is used that allows it to go "higher"... maybe.. or something is happening to cast the value back and forth between the internals of the game and the UI... or maybe its part of the "saving the player map" process that shrinks/corrupts the value.... I have literally no idea, but for whatever reason, you can put in "a lot" (like 1 million) in the forge, and something, somewhere does not expect this and it appears it doesn't show up until the game is saved + restarted (from my testing). Maybe in multiplayer it causes other issues or behaves differently.
The end result is: The max forge units for whatever reason, may cause "unstability" to go above a certain number, at least in a19. This mod was developed for a18 originally (maybe earlier?) so maybe things have changed inside the game since then and it is no longer suitable for a19+. I have tested it before and didn't get these results, but then again, I wasn't doing testing this deeply nor had specific mods to mimic the system having issues.
Special thanks to
@cereza1985for bringing this to my attention, and providing more information to replicate! There have been some earlier reports of issues with this mod and I was unable to replicate at that time on my system, and the game versions have changed as well (and mods, etc) so it appears the earlier reports may also have been valid for earlier a19 game versions.