EVERYONE!
There was a name collision with another modder's mod, overwriting a file in my mod. I had to rename that file and all references to it.

When you build something in Transport Fever, the current mod references are saved with the build. So now when a file has been renamed, all references in your save game are now invalid (they point to the older version).

I did a step-by-step instruction below so the save game can be loaded again, then you need to rebuild (might be enough to change the options and hit apply) all harbors to get all the references updated to the updated mod. This is how Transport Fever works, not much I can do about it.

I'm still in the testing phase, so please give feedback if the instructions worked. Here they are again to be clear:

1. Copy the file "cargo_pier_3_right_through.mdl"
2. Rename the copy to "cargo_pier_3_through.mdl" (Note, the original file must still be intact)
3. Load your saved game and rebuild all harbors made with this mod (it might be enough to change one of the options and hit apply)

There might be a need to also do the same procedure with "cargo_pier_1_right_through.mdl" to get it all to work. I will put up a pinned instruction when we have this confirmed.

The mod is located in your steam game library (the root might vary depending on your installation):
C:\Program Files (x86)\Steam\steamapps\workshop\content\446800\805490341\res/models/model/station/harbor/cargo/

Hello Gwinda
I was following your process and was now able to start the game. After rebuilding my harbors I saved the game and shut down it. A restart was possible without any issue yet. I did not change any at "cargo_pier_1_right_through.mdl".

Thank you, I just released a Compatibility patch doing all this.
The "cargo_pier_1_through.mdl" is only needed if you have this configuration in your old game save.