Out of the blu, today i've got this error on save loading:

The mod Underground heat pipe (2.2.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event underground-heat-pipe::on_configuration_changed
underground-heat-pipe/control.lua:58: attempt to index local 'old_entity' (a nil value)
stack traceback:
underground-heat-pipe/control.lua:58: in function 'bring_heatpipe_to_front'
underground-heat-pipe/control.lua:285: in function 'check_for_neighbours'
underground-heat-pipe/control.lua:294: in function 'update_mode_for_all_pipe_to_grounds'
underground-heat-pipe/control.lua:148: in function <underground-heat-pipe/control.lua:143>

Any idea how to get around this?

that's not a startup error but a save load error

and it's not out of the blue since you updated the version of factorio, one or more mods, or added/removed a mod

Well, it's not a mod update, i've rolled back all updates in the last few days and i still got the error. However a previous save is working, i think before using underground hp in factorissimo. Any ideas?