This bug is definitely reproducible for me, and always happens when i interact any ways with Auto heavy sieve. Outcome of any action is that singleplayer client freezes, and has to be forced shutdown.

Possible interactions that freeze game:

• Right click the sieve in attempt to open UI, can instantly freeze game.
• Insert a stack of compressed gravel and wait a few seconds. With this i can tell that the animation from mini-player inside machine may not be exactly in sync with what UI tells. If you keep opening UI it might freeze the screen right there, but if you keep waiting it will still freeze the game in less than a minute. I had F3 screen open and used RAM was moving in healthy range 2-3GB. Log files seem useless for this case, memory leaks maybe but no hints as to why.
• Some case was that i had 4 sieves powered with RF and then added 5th unpowered with also diamond mesh inside. None of them had anything to sieve inside, and yet it some minutes later froze my game. I had never frozen before i started playing with heavy autosieves, and i have now reproduced that for about 8 times.

latest.log

[23:56:08] [Server thread/INFO]: Zaflis joined the game
[23:56:12] [Client thread/INFO]: [CHAT] InvTweaks: Configuration loaded.
[23:56:14] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 6448ms behind, skipping 128 tick(s)

fml-client-latest.log

[23:56:42] [Server thread/INFO] [FML/]: Unloading dimension 1
[23:56:55] [Server thread/DEBUG] [FML/]: Gathering id map for writing to world save Skylands
[23:57:30] [Server thread/DEBUG] [FML/]: The world 3edb8093 (Skylands) may have leaked: first encounter (5 occurrences).
[23:57:30] [Server thread/DEBUG] [FML/]: The world 3b1ee396 (Skylands) may have leaked: first encounter (5 occurrences).
[23:57:30] [Server thread/DEBUG] [FML/]: The world 20ae836d (Skylands) may have leaked: first encounter (5 occurrences).
[23:57:40] [Server thread/DEBUG] [FML/]: Gathering id map for writing to world save Skylands
[23:58:20] [Server thread/DEBUG] [FML/]: The world 3edb8093 (Skylands) may have leaked: seen 10 times.

I'm playing FTB SkyFactory3 modpack for Minecraft 1.10.2 which has ExCompressum_1.10.2-2.0.68.jar . Game has 4.5GB of RAM set for max value.

Can you reproduce this with only Ex Compressum (+ Ex Nihilo) installed?
Does the same happen to the normal Auto Sieve?

Normal auto sieve was fine before. I updated the modpack to latest and i can't replicate the bug in the multiplayer save anymore. Also couldn't reproduce on a new world in creative. Maybe it was a mod interaction with something in my base, and that would be painful to find out what it was.

Alright, we'll see if it pops up again. The normal Auto Sieve and the Heavy Auto Sieve share 99% of the same code, so it was most likely something else causing the issue.