I created a new simulation project called 'Diffuser':
Alternate design for a diffuser that, hopefully, will allow it to be much shorter than a traditional design.
More of my public projects can be found here.
I took a quick look at your failed sim. I’m pretty sure it’s due to you boundary condition setup - you chose a Velocity Inlet condition but you’re sucking the fluid out of that face (see positive z direction). So either make it a Velocity outlet or, even better (=more stable) drive the flow over the other side, what you currently have defined as a pressure outlet.
Hope that helps,