Got this crash on loading an existing multiplayer save after upgrading a few random mods that have updated recently, including this one (and muluna and the enable all planet had an update, not much else that I recall), and the machine-upgrades mod says to remote its crashes here:

The mod Machine Upgrades (0.69.12) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event machine-upgrades::on_configuration_changed
Error when running interface function machine-upgrades-techlink.add_upgrade_data: Error when running interface function machine-upgrades-techlink.add_technology_effect: __machine-upgrades__/script/beacon-manager.lua:38: Something stopped us from making a special beacon. Alert the mod creator of how this happened.
stack traceback:
    [C]: in function 'assert'
    __machine-upgrades__/script/beacon-manager.lua:38: in function 'try_get_beacon'
    __machine-upgrades__/script/beacon-manager.lua:50: in function 'update_entity_moduling'
    __machine-upgrades__/script/beacon-manager.lua:108: in function 'to_call'
    __machine-upgrades__/script/entity-modifier.lua:64: in function 'try_apply_modifier'
    __machine-upgrades__/script/entity-modifier.lua:77: in function 'apply_auto_modifier'
    __machine-upgrades__/script/technology-linker.lua:221: in function <__machine-upgrades__/script/technology-linker.lua:181>
stack traceback:
    [C]: in function 'call'
    __machine-upgrades__/script/technology-linker.lua:254: in function <__machine-upgrades__/script/technology-linker.lua:248>
stack traceback:
    [C]: in function 'call'
    __machine-upgrades__/script/automatic-tech-linker.lua:59: in function 'config'
    __machine-upgrades__/script/event-lib.lua:124: in function <__machine-upgrades__/script/event-lib.lua:123>

I notice it mentions moduling and beacons, we did just start using beacons (started modules a couple days ago), on nauvis only so far (only traveled from lignumus and got to muluna yesterday on prior save). Currently the server is unloadable with the existing save and trying to load the save in the clients gives the same error in the GUI in-game and fails back to the main menu.

Thank you for the report. I am sorry to hear that you are having this issue. I have also received a similar report on discord. I need to update the error message to give more details.

Basically, Machine Upgrades functions by placing invisible beacons to upgrade specific entities. This is how I can make a technology effect like “all crushers -20% power” or whatever. The issue here is that the moment Machine Upgrades is making this beacon (the mupgrade beacon), another mod is listenning to the build event and destroying it before I even get a chance to use it. I’ve spoken with the other mod devs on this topic before, and there is unanimous agreement that the other mod is at fault for destroying my machine automatically in an on_built_entity event.

Unfortunately, because of the way the code works, Machine Upgrades cannot figure out which mod is causing this major game-breaking incompatibility.

My recommendation to you is to try to find out which mod(s) were updated to figure out which modder is responsible for destroying this beacon (likely on an on_built_entity event callback). Let me know if you find anything out, but I likely can’t do anything on my side besides talking with the other modder. Given that I’ve received several identical reports back to back this morning, it is likely a mod that has been recently updated.

Update: Try downgrading your version of Muluna to see if that fixes your issue.

Apparently the latest Muluna bug turns all space platform entities into radars, which will definitely cause MUpgrades to crash.

This has been formally confirmed to be caused by the new Muluna update. I recommend downgrading Muluna until MeteorSwarm can make a proper fix.

FYI even if you uninstalled Rubia to circumvent the crash, this Muluna bug apparently turns all machines on space platforms into satelite radars…. So you probably don’t want to open and continue on the save anyway.

As such, this is not a Rubia/MUpgrades bug. MUpgrade consistency checks are successfully “failing” on this Muluna bug (and they should fail given what Muluna is doing!). In this case, the consistency checks prevent Muluna’s code from wrecking your save.

Basically, Machine Upgrades functions by placing invisible beacons to upgrade specific entities. This is how I can make a technology effect like “all crushers -20% power” or whatever. The issue here is that the moment Machine Upgrades is making this beacon (the mupgrade beacon), another mod is listenning to the build event and destroying it before I even get a chance to use it.

That is a wonderfully simple method of doing such upgrades, the prior method I've seen a mod do it is by spamming prototype variants out, which was... slow...

I’ve spoken with the other mod devs on this topic before, and there is unanimous agreement that the other mod is at fault for destroying my machine automatically in an on_built_entity event.

Yeah I entirely agree, that seems... odd for someone to do... The only way I could think of this mod being at fault were if the invisible radars were not able to be placed on space platform or so (if only Factorio's API returned an error structure on failure with more information...), but that seems exceedingly unlikely unless something is changing the radar's tile or surface flags...

Apparently the latest Muluna bug turns all space platform entities into radars, which will definitely cause MUpgrades to crash.

That... seems ... an odd thing to happen... o.O
Hmm... There's been a lot of commits since the last version I ran, apparently it updated to a 2.0.0 (then up through and to 2.0.2 in the same day)...
Looking over the code for anything that even hints at a radar and I'm not running across anything odd yet in the lua... wonder if it's an interaction with another mod somehow...

This has been formally confirmed to be caused by the new Muluna update. I recommend downgrading Muluna until MeteorSwarm can make a proper fix.

Will do, hopefully the clients will sync properly to the server then, thanks much! Where was this formally confirmed at so I can follow the updates?

FYI even if you uninstalled Rubia to circumvent the crash, this Muluna bug apparently turns all machines on space platforms into satelite radars…. So you probably don’t want to open and continue on the save anyway.

Understatement, lol...

As such, this is not a Rubia/MUpgrades bug. MUpgrade consistency checks are successfully “failing” on this Muluna bug (and they should fail given what Muluna is doing!). In this case, the consistency checks prevent Muluna’s code from wrecking your save.

Exceedingly useful, stability checks are not valued highly enough in general.

Might mess more into muluna's code to see if I can find the root cause if I get time today, thanks for the lead!

Muluna just released 2.0.3 (and 2.0.4 and 2.0.5 for other issues) and it states:

Bugfixes:
- Fixed that when any entity would be placed on a space platform, a satellite radar would be placed instead.

Updating and attempting to launch server...

And success! Other mod fixed their bug, it ended up just being an inverted ==/~= check, gotta love those, lol. Associated commit:
https://github.com/nicholasgower/planet-muluna/commit/f6c32a562ef9715ac42e3b8de2acf2efcf5fdbc8