Runaway loop detected

I am making a boardgame, with about 100*100 cells on board.

But when I use two loops to update the status of cells, the following error occured.

Runaway loop detected (over 1000000 iterations)

The board is about 50*70 at that time(the number of cells increase during game), I think it’s far from being over 1000000 times.

Any hint will be welcome.

You can change maximum number of iterations in engine settings, but for 3500 elements it is strange that cycle is breaking with runaway.
Have you debugged your blueprint? Try to print array size to screen, or save size to variable, maybe it is greater than you thought.
Some times you can just forget that you did something that might to be changed later. Once I had something like that with my FPS game: I was forcing player to spawn in (0,0,0), but totally forget about that and about 3 weeks I was trying to spawn player position to where he supposed to spawn. Then i rewrote code and found that mistake.