Just one single change made this model not printable

 

I made a tablet stand that can produce three angles.
 
It was beautifully printed and worked well. 

back side

front side

tablet placed in the front

 

tablet placed in the back

tablet lifted with leaning angle

lifted tablet

However, the tablet may slip from the back.
slip is not happening at the moment. but to make it more stable. I wanted some solution.
 
so,
A cube block was added in the back side to prevent slipping.

 
 

and wanted to print it. 

but when I import 'stl' file to the slicer, error occured. 

 

so, I brang previous one, and compared. 

only the single change made this object not printable. 

 (Actually we can print it, if we succed to generate Gcode. but in the printing process, some kind of problem such as detachment arises. so, better work without the error message.) 

 
I don't know why. 
 

maybe some operation between objects (hidden or not) 

created collision of coding? 

somebody tell me.