This is brand new after latest update.
When trying to use the update blueprint button, from a blueprint in a book, and selecting a build with a loader in it. Does not need to be a filter loader despite it saying something about that, it was happening with chutes. Doesn't seem to happen if you use a blueprint straight from your inventory, only from book (or book in library).
Will update if I get more details.

7845.447 Error MainLoop.cpp:1285: Exception at tick 6343736: The mod Miniloader (1.14.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event miniloader::on_player_setup_blueprint (ID 69)
miniloader/lualib/blueprint.lua:160: attempt to index local 'bp' (a nil value)
stack traceback:
miniloader/lualib/blueprint.lua:160: in function 'filter_miniloaders'
miniloader/control.lua:326: in function 'handler'
miniloader/lualib/event.lua:15: in function <miniloader/lualib/event.lua:13>

If you fixed the orientation issues on miniloaders in this update then thank you very very much, that's been driving me nuts, that and the wires.

Just ran into this as well when trying to select new contents for a blueprint!

15117.157 Error MainLoop.cpp:1285: Exception at tick 29910846: The mod Miniloader (1.14.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event miniloader::on_player_setup_blueprint (ID 69)
__miniloader__/lualib/blueprint.lua:160: attempt to index local 'bp' (a nil value)
stack traceback:
        __miniloader__/lualib/blueprint.lua:160: in function 'filter_miniloaders'
        __miniloader__/control.lua:326: in function 'handler'
        __miniloader__/lualib/event.lua:15: in function <__miniloader__/lualib/event.lua:13>
15117.157 Error ServerMultiplayerManager.cpp:91: MultiplayerManager failed: "The mod Miniloader (1.14.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event miniloader::on_player_setup_blueprint (ID 69)
__miniloader__/lualib/blueprint.lua:160: attempt to index local 'bp' (a nil value)
stack traceback:
        __miniloader__/lualib/blueprint.lua:160: in function 'filter_miniloaders'
        __miniloader__/control.lua:326: in function 'handler'
        __miniloader__/lualib/event.lua:15: in function <__miniloader__/lualib/event.lua:13>"
15117.157 Info ServerMultiplayerManager.cpp:780: updateTick(29910846) changing state from(InGame) to(Failed)
15117.157 Quitting: multiplayer error.

Should be fixed as of 1.14.4.