I just updated to the latest version to fix the underground pipe length and ran into this issue.
Is there any other information that I can get for you to help determine the cause of it?

349.293 Error ServerMultiplayerManager.cpp:91: MultiplayerManager failed: "The mod Fluid Must Flow (1.2.14) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event FluidMustFlow::on_robot_built_entity (ID 14)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function '__index'
__FluidMustFlow__/linver-lib/entity-utils.lua:66: in function 'addNeighbourPositionDataOf'
__FluidMustFlow__/linver-lib/entity-utils.lua:22: in function 'getConnectedNeighbours'
__FluidMustFlow__/control.lua:140: in function 'checkFluidboxConnections'
__FluidMustFlow__/control.lua:116: in function 'ductsConnectionsControlCallback'
__FluidMustFlow__/control.lua:390: in function <__FluidMustFlow__/control.lua:384>"

This happened on Factorio v1.1.6. The mods that were loaded at the time are

AutoDeconstruct_0.1.21.zip
Bottleneck_0.11.7.zip
Construction_Drones_0.6.11.zip
FNEI_0.4.0.zip
FluidMustFlow_1.2.14.zip
Krastorio2_1.0.18.zip
LogisticTrainNetwork_1.15.0.zip
ModuleInserter_5.2.2.zip
YARM_0.8.201.zip
aai-industry_0.5.3.zip
aai-signal-transmission_0.4.1.zip
alien-biomes_0.6.3.zip
crafting_combinator_0.16.1.zip
factoryplanner_1.1.11.zip
flib_0.6.0.zip
informatron_0.2.1.zip
jetpack_0.2.5.zip
krastorio2_extended_endgame_1.0.0.zip
robot_attrition_0.5.7.zip
space-exploration-graphics_0.5.4.zip
space-exploration-hr-graphics_0.5.1.zip
space-exploration-postprocess_0.5.6.zip
space-exploration_0.5.26.zip

The issue went away after a restart, so it seems that it does not happen every time.