I apologise in advance for not isolating this issue properly, I simply do not have the time, but I can give exact steps to reproduce. If you download the Floramancer pack on Twitch, which includes Botania, Garden of Glass and Quark, unlock the mod versions, update Forge to latest and update all mods but Quark to latest (so they don't complain about change in Forge version), through Twitch/Curse, and attempt to start a new Garden of Glass world, all is well.
However if you then update Quark through Twitch/Curse, currently resulting in v148, creating a Garden of Glass world will hang on 'loading world'. Manually replacing the Quark version in the instance files with v147 will also cause this problem.
Replacing it with v146 or lower fixes it however.
I know this is an issue in a modpack with some CraftTweaker scripts and so on, but it seems clear to me that this is at least a potential issue in Quark itself, as it is fixed/broken purely via a Quark version change. I hope this report is somewhat useful.

Edit: FYI, the log files display no sign of a problem, and when Minecraft is force closed (as it has hung), they simply state the world was saved as normal.

Edit 2: Everything you do vazkii is genius, thanks for hours of fun!

Ugh, I hate hangs. They're annoying because it's so hard to find exactly what causes them...

This also happens for me when attempting to generate a void world with the Void Island Control mod, and is resolved similarly by removing Quark to do the initial world-gen. I don't think CraftTweaker is the issue, as I don't have it loaded on my own setup.
Restoring Quark once the void world has been created appears to cause no issues and allows the world to load as normal, however.

Sorry if I'm way off, but could it be the while loop in ArcheologistHouseGenerator.java, current on line 41, which repeatedly looks downwards for a non air block, but would never find one in a void world?

Additional info: A void world that was generated with Garden of Glass, but without Quark, and then reloaded with Quark returned, eventually has a soft-hang possibly related to this. By soft-hang I mean you can still move about and break blocks, but no drops happen, and I can't dig up pebbles. Game will lock up entirely if I try to save and quit to menu.
Furthermore, if I then kill java and attempt to reload the world again, nothing is rendered on reload except for sky, sun, moon and clouds - no player, terrain or anything else will render, not even the block selection box or Hwyla.
Log seems to suggest there's a leak, so perhaps where it's doing the Archeologist search as described above, it's using more and more memory to try to find it until these symptoms occur, thus causing a hang there too?

Semi-confirmed my suspicions, disabling the Archeologist in config fixes the issue.

Additional info: A void world that was generated with Garden of Glass, but without Quark, and then reloaded with Quark returned, eventually has a soft-hang possibly related to this.

Ditto! I also experienced the "soft hang" after re-adding Quark to my GoG world, and also the hard lock-up when trying to save and quit. (At least now I can stop tearing my hair out wondering what went wrong!)