Crashes when using a blueprint (from a bit ago) .

The mod Miniloader (1.15.6) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event miniloader::on_robot_built_entity (ID 14)
miniloader/circuit.lua:83: attempt to index field 'split_lane_configuration' (a nil value)
stack traceback:
miniloader/circuit.lua:83: in function 'sync_behavior'
miniloader/lualib/miniloader.lua:156: in function <miniloader/lualib/miniloader.lua:137>
(...tail calls...)
miniloader/control.lua:112: in function 'handler'
miniloader/lualib/event.lua:15: in function <miniloader/lualib/event.lua:13>

Also, crashes ever a loader is used. not just a blueprint

Hmm, not sure how the mod could get into that state, but you should be able to fix it in the console by entering the command:

/c __miniloader__ global.split_lane_configuration = {}

If you haven't used a console command in that game previously it will warn you that using it will disable achievements for this game and make you enter it again to actually run it.