i wanted to place the default cube into the output of the recovery bay and the game crashed.
i had the recovery bay connected to a boiler via inserters, if that helps.

here is the error message:

The mod Ultracube: Age of Cube (0.5.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Ultracube::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__Ultracube/scripts/cube_search.lua:386: in function 'cube_search_inserters'
Ultracube/scripts/cube_search.lua:547: in function 'cube_search_local'
Ultracube/scripts/cube_search.lua:901: in function 'update'
Ultracube/scripts/cube_fx.lua:565: in function 'tick'
Ultracube/control.lua:318: in function <Ultracube/control.lua:316>

just got another error, this time without doing anything actively.
with the same error message.
it should be another recovery bay-to inserter-to some production building interaction that caused the issue.

this looks like a possible mod interaction, though i don't think any of my mods can cause problems with inserters.

ok, i caught it in action and can semi-reliable repeat it.
it happens when the inserter drops the cube into a machine or grabs it out of one.
i have caught it happening with boilers, synthesizer and ultra dense furnace, all in combination with a recovery bay and fast inserters.

i have looked through my mods, there is nothing that can cause this issue.
none of the mods affect inserters, besides bobs adjustable inserters, but that mod has never caused a crash for me before.

i also got a slightly different error message during the tests:

Error while running event Ultracube::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__Ultracube/scripts/cube_search.lua:338: in function 'cube_search_transport_lines'
Ultracube/scripts/cube_search.lua:546: in function 'cube_search_local'
Ultracube/scripts/cube_search.lua:901: in function 'update'
Ultracube/scripts/cube_fx.lua:565: in function 'tick'
Ultracube/control.lua:318: in function <Ultracube/control.lua:316>

could this be an issue caused by the cube tracker or the cube explosion animation, that happens if it's triggered the moment the cube goes into the machine?

The problem is almost certainly an incompatibility with another mod you are using. It doesn't have to be a mod that affects inserters, it could be anything that is secretly deleting/recreating entities. Try disabling other mods and running /cube_refresh in the console, odds are the problem will not persist. Please post your mod list.

the command worked.
for about 3min...
this time the inserter dropped the depleted cube onto the ground and it crashed the game.
same error message.

here is my full Mod list:

AAI Loaders
actual crafting time
avanced fluid handling
big brother
big data string library
blueprint flip and turn
bob's adjustable inserters mod
bob's functions library mod
bottleneck
crafting priority
disco science
doing things by hand
electric furnaces
enable research queue
even distribution
factorio library
factorio standard library
fill4me
flare stack
slare stack speed tweak
flow control
ghost in hand
krastorio 2 assets
loaders snap, work and utils
long and far reach
max rate calculator
merging chests all types
milestones
miniloaders
nanobots:early bots
nearby ammo count
not so simple wood
omega drill
picker atherneum
picker dollies
quarry (updated for 1,1)
recipe book
remote configuration
scaredy bots
simulation helper
squeak through
teleporters
the big furnace
the ruins mod
turrets drop ammo
ultracube:age of cube
upgrade machines
vehicle snap
waterfill
wire shortcuts

i narrowed the issue down to the following mods, (names from mod sync list between the saves, so name formatting is a bit different)

VehicleSnap
WideChestsAllTypes
Actual_Craft_Time
blueprint_flip_and_turn
EnableResearchQueue
Teleporters
upgrade-machines

my guess is on the chest mod, even if no chests were involved.
but i'm waaay too sleepy to continue testing mod combinations.
i hope this helps enough.

Yep that command will only fix things so long as there is no problematic mod to break things again.

Wide chests causes this crash if it is not the latest version

it should be the latest version, or rather that specific mod has had not updates at all as it is just an additional settings mod for the actual merging chests mod.
so there is still a chance that there is weird interactions.

the actual merging chest mod i couldn't use together with squeak through and ultracube for some reason. so only the merging of all types mods stayed active, which was useless alone.
until i just now reset the actual merging chests mod to default.

...ok i think narrowed that problem down to one weird interaction.
when using ultracube together with merging chests, the squeak through mod doesn't like the merging chest settings for any chest other than the default steel chest merges.
if i change anything for wooden or iron chests the mods won't load together.
and since the merging all types additional mod changes exactly those settings, the actual merging chests mod was disabled while the additional merging chests of all types mod was not, which is probably what caused this whole crashing game situation.

i guess i'll play ultracube with only one of these mods for now...