After I changed settings of Merging chests and installed "Merging chest unlimited", the exception appeared on any using of a Container UPS+ tool.

the whitelist on Merging chests setting:
1xN Nx1 2x2 2x3 2x4 2x5 2x6 2x7 2x8 2x9 2x10 3x2 4x2 5x2 6x2 7x2 8x2 9x2 10x2 3x3 3x4 3x5 3x6 3x7 3x8 3x9 3x10 4x3 5x3 6x3 7x3 8x3 9x3 10x3 4x4 4x5 4x6 4x7 4x8 4x9 4x10 5x4 6x4 7x4 8x4 9x4 10x4 5x5 6x6 7x7 8x8 9x9 10x10 11x11 12x12 13x13 14x14 15x15

The mod Big Containers UPS+ (1.0.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event BigContainersUPSPlus::on_player_selected_area (ID 51)
Unknown entity name: high-steel-chest-3-UPSPlus
stack traceback:
[C]: in function 'create_entity'
BigContainersUPSPlus/control.lua:125: in function 'replace_container_entities'
BigContainersUPSPlus/control.lua:148: in function <BigContainersUPSPlus/control.lua:147>

Hi Flart, happy to see you are still enjoying the mod :)

I tested this out and was getting the same error (thank you for the details!). I uploaded a new version which should have fixed the issue.

For the curious - this mod was doing it's duplication work before the other mod actually added it's chest variations. No clue why this only started happening now.