When I try to load a world a few seconds later that the world is loaded th emod gives me this error:

"
Error while running event research-queue-the-old-new-thing::on_nth_tick(60)
...rch-queue-the-old-new-thing__/functions/update_queue.lua:229: attempt to get length of field '?' (a nil value)
stack traceback:
...rch-queue-the-old-new-thing__/functions/update_queue.lua:229: in function 'update_queue_force'
research-queue-the-old-new-thing/control.lua:286: in function <research-queue-the-old-new-thing/control.lua:283>
"

This sounds an awful lot like the issue with the will o' the wisp mod... I was meaning to write up a test case for this and make sure it is working but didn't get around to it yet. Did you have any other mods installed in this game? Were you loading RQ into an existing game? Any further details will help to get a clearer picture of the issue.

Yeah actually it was a mod that make this crash. If I remember rightly it was the mod that adds the train constructor. I uninstalled it and this mod now works perfectly. I have a lot of other mods and it doesn't give me errors.

No. The RQ was in this savegame when I created it.

I was overcomplicating things. I didn't even realise that I hadn't released the intended fix for the issue with the will o' the wisp mod. A unit test for this would be good to have but in the meantime the v1.7.6 release should fix it.