Thank you for the reply!
Error (when invisible):
E 0:00:15.357 _input: Condition "!is_visible_in_tree()" is true.
<C++ Source> scene/2d/touch_screen_button.cpp:201 @ _input()
<Stack Trace> LEVEL_1.gd:83 @ _process()
Again, what I suspect is that it is constantly under check, and when it doesn't see it (if it is invisible) it throws error message. But the game doesn't crash or anything, it works fine. But debugger shows hundreds of these errors (probably every frame?)