I have produced 9 flesh and put it into the Fisher then this Error Massage occured:

Die Mod Cursed Upgradable Buildings hat einen nichtbehebbaren Fehler verursacht.
Bitte informiere den Modautor über diesen Fehler.

Error while running event Cursed-Exp::on_tick (ID 0)
Cursed-Exp/scripts/levelEntity.lua:97: attempt to index local 'ret' (a nil value)
stack traceback:
Cursed-Exp/scripts/levelEntity.lua:97: in function 'changeEntity'
Cursed-Exp/scripts/levelEntity.lua:75: in function 'fishers'
Cursed-Exp/scripts/on_tick.lua:226: in function 'main'
Cursed-Exp/control.lua:194: in function <Cursed-Exp/control.lua:192>

That's weird, i test it in a lot of ways, never see that error.
Can you tell me others mod are you using?
Only with fisher?

I havent tested it further, because when i reload the crashed save game i got another error log. But the autosave is overwritten will put it here when i have set it back up. My base is up to 20% run by robots so i dont have the time to react before the flesh get taken to the fisher.

Modlist taken from Mod-list.json. Your mod is no longer in this list because its deactivated.

AAI-CraftTab
aai-programmable-vehicles
aai-signals
aai-vehicles-hauler
aai-vehicles-miner
aai-zones
AdjustableSciences
Advanced_Machines
alien-biomes
alien-biomes-hr-terrain
alien-module
AllFurnaceswithModules
Bio_Industries
bobassembly
bobinserters
boblibrary
boblocale
bobmining
Bottleneck
cncs_Sulfur_Mod
Compound_Splitters_Continued
deadlock-beltboxes-loaders
Electric Furnaces
ElectricTrain
Factorissimo2
FARL
fish_slurry
Geothermal
LootMilitaryScience
Nightfall
Noxys_Waterfill
OmegaDrill
OpteraLib
Power Armor MK3
Power_To_Liquid_updated
quarry
ReStack
reverse-factory
robotworld
Robot_Battery_Research
robot_tree_farm
scattergun_turret
SchallAlienLoot
SchallAlienMutation
SchallAlienTech
SchallAmmoTurrets
SchallBeltConfiguration
SchallEndgameEvolution
SchallGunPod
SchallLampContrast
SchallRechargingWeapon
SchallRemainsPickup
SchallTankPlatoon
Single-Splitter-new
SolarWalls
sonaxaton-infinite-resources
stdlib
StorageTank2
Swarmageddon
Texugo_windgenerator
Turret-Shields
VehicleGrid
VersepellesAquaFarm015
Warehousing
WeakEnemies
Wood_Gasification

edit:
The Mine evolved from 1 to 2 when i put the rest of the Meat into the Fisher i get again the error message:
Die Mod Cursed Upgradable Buildings hat einen nichtbehebbaren Fehler verursacht.
Bitte informiere den Modautor über diesen Fehler.

Error while running event Cursed-Exp::on_tick (ID 0)
Cursed-Exp/scripts/levelEntity.lua:97: attempt to index local 'ret' (a nil value)
stack traceback:
Cursed-Exp/scripts/levelEntity.lua:97: in function 'changeEntity'
Cursed-Exp/scripts/levelEntity.lua:75: in function 'fishers'
Cursed-Exp/scripts/on_tick.lua:226: in function 'main'
Cursed-Exp/control.lua:194: in function <Cursed-Exp/control.lua:192>

Can confirm this crash. Also a weird issue with the fishers where if you put one down, put a tank close to it, and try to connect them with a single piece of pipe, it says "Can't mix fluids" It appears it thinks the fishers are putting out water like regular pumps. just pulling a pipe across from it works fine though.

https://i.imgur.com/CT6JwjM.jpg

Mine won't even upgrade after putting in the flesh, Also fishers started putting out blood again since I switched to latest experimental build (0.17.x) Had to replace them though

Are you using 0.6.5? I can't reproduce it.
Can send me the save and mods to my discord? https://discord.gg/6VVxuVH
Thanks