I’m totally new to UE so please be lenient 
I’m working on my first, very simple game, with two “characters” (they are just a blocks ) between which we can switch any time and two levels. Before I made a second level everything was ok, but now, in this level when I start to play and then exit, I’m spammed with errors of some complicated blueprints of which existence I didn’t know up to this moment 
Everything seems to be ok with these blueprints, as I can compile and save them with no problem, but it’s not fixing these error logs. What’s the problem?
Bascially, click on the far right item in the error line ( in your case, the first two are SetOrthoWidth and SetWorldLocation ) and it will take you to the offending code.
In the code, at that point, the node will taking input from a reference to something ( an actor / material etc ). That reference is pointing at nothing. In other words, you have not set it to anything…
I get what you’re talking about, but I see no nodes pointing at nothing. Besides, it’s some base UE blueprint, I haven’t touched this at all, why it’s bugged now?
I still can’t fix this 
Everything is ok in level 1, problem occurs in second one. The seemingly bugged blueprint is some base UE stuff as I suppose, that I do not fully understand, I have not touched this, and I can’t find any “references pointing to nothing” in there. I can compile and save it, but this doesn’t fix anything. Any ideas?
These are the most common errors you can get. It means the engine is pulling from a variable that is Null (hasn’t been set to anything yet). Sometimes this can happen even when your logic is more or less good but there is a brief moment in time where the variable hasn’t been set yet.
Use the “Is Valid” node to stop the flow of logic if a variable hasn’t been set. This can solve a lot of these types of issues. This is the Is Valid Node" unreal engine is valid - Bing
It means, it cant read the first variable, then, how is it going to use it? so everything else fails. just click the zoom symbol to find the node wich is not understanding what to do.
Guys, I
Don’t know if you get me right: guys. I have not made this bugged blueprints shown in log 
I know where are these broken nodes, but I have no idea how do they work exactly and they are all connected to something.
Look at this:
I assure you, I’m not capable of making such advanced blueprints… yet 
Everything is good in level one, level 2 has these logs even with blank level blueprint.
Can’t find what’s wrong I can make and test some new stuff, but it’s really irritaiting to see these error logs everytime I quit game mode and return to editor. How can I fix something that’s not my creation and I don’t know how it works?