I asked over at Reddit first, in hopes someone could help me, but nothing so far (after about 12 hours).
So now I’m here, and I’m really really hoping someone can help me, or hours of work has to be lost 
Whenever I try to save, I get this Error message. When I ask it to check into it further, I get the second picture.
I don’t know what to do with this. I can’t save it, and a lot of work had gone into it before I noticed the issue (I’m also pretty sure the problem disappeared suddenly at one point, and then came back again later). If I have to close it down now, without saving it, I’m losing hours of work, and stuff I barely just figured out how to do, and a bunch of minor fixes I was doing with level layout. It would really suck. I’m really hoping any of you here can give me some insight into what could be causing this, and how I can fix it so I can save the darn project!