Reinstalled Factorio recently and found Heavily Modded Modpack and played a while last weekend. It's amazing to have so many mods in a single package that mostly work without a hitch.
Tested and replicated on fresh Factorio installation with only ever the modpack installed; Factorio files verified via Steam.

I'll file bug reports with the projects in question as well, but wanted to provide info to you in terms of looking into whether to temporarily exclude those mods.

That said:
Bug: Having Fill4Me breaks starting a game from the Map Editor or a saved scenario.
Steps to reproduce:
Start game with full modpack, including Fill4Me loaded. Go into Map Editor -> New Scenario -> Freeplay -> generate a map with arbitrary settings -> Select "Save and Play".
This will result in a non-recoverable error. Saving the scenario and attempting to start it from the scenario selection does not work either.

Bug: Ruins Mod and/or Ruins Mod Krastorio cause reproducible non-recoverable errors when spawning in certain ruins.
Steps to reproduce:
As above: fully load modpack including Ruins Mod and Ruins Mod Krastorio. This can then be reproduced by just playing the game and exploring the map or exploring the map in the Map Editor. It's somewhat rare, but still quite frustrating. The error can be reliably triggered by loading the Ruins Map debug map from the scenario selection.