Piece checkmates 1 practice engine stuck

I tried to practice piece checkmates. The engine got stuck in the two rook mate. I tried it a second time and the computer keeps thinking...

To repeat it this behaviour yourself try:

1. Rad1+ Kc5
2. Rh5+ Kc6
3. Rc1+ Kd6
4. Rcc5 Ke6
5. Rh6+ ...

Is this a bug?

EDIT: Third try exact same behaviour.

Does it say WASMX?