MeGUI occasionally gives me problems too. That's why I use the command-line. I always download the latest version from here.

This is the sort of command-line I use.

x264 --pass 1 --bitrate xxxxx --bluray-compat --level 4.1 --preset slow --tune film --keyint 24 --sar 1:1 --slices 4 --vbv-bufsize 30000 --vbv-maxrate 40000 --b-pyramid none --weightp 0 --colorprim bt709 --transfer bt709 --colormatrix bt709 --merange 24 --stats ".stats" --output NUL "onput.avs"
x264 --pass 2 --bitrate xxxxx --bluray-compat --level 4.1 --preset slow --tune film --keyint 24 --sar 1:1 --slices 4 --vbv-bufsize 30000 --vbv-maxrate 40000 --b-pyramid none --weightp 0 --colorprim bt709 --transfer bt709 --colormatrix bt709 --merange 24 --stats ".stats" --output "output.264" "input.avs"

You can use the slower profiles if you like.

Re: --open gop

If the switch is not in the command-line, the GOPs are closed.

Re: --tune grain

It is not usually recommended for BD, so use --tune film.