Geometry stitch failed warning in HFSS

Hi all,
I am doing HFSS simulation for combination of FR4 PCB and Metal PCB. My simulation failed after running several hours. There was a warning message-"Geometry Stitch Failed. Falling back to backup process".

I'm using HFSS 15.0 (64 bit) for analysis.
Can anyone give me idea what is this warning message and how to get rid of this?

I have the same warning following by 2 errors
HFSS 15 x64 too