Bug#458059:

["Stijn van Drongelen" <tinctorius@gmail.com>]

The issue seems to have its roots in build/source/texk/web2c/tex.web,
line 5925-5930:

if cur_level=max_quarterword then overflow("grouping levels",
@:TeX capacity exceeded grouping levels}{\quad grouping levels@>
  max_quarterword-min_quarterword);
  {quit if |(cur_level+1)| is too big to be stored in |eqtb|}
cur_boundary:=save_ptr; incr(cur_level); incr(save_ptr); cur_group:=c;
end;

So if 'cur_level' exceeds the maximum quarterword size, TeX dies.
These constants are defined in 2411-2414:

@d min_quarterword=0 {smallest allowable value in a |quarterword|}
@d max_quarterword=255 {largest allowable value in a |quarterword|}
@d min_halfword==0 {smallest allowable value in a |halfword|}
@d max_halfword==65535 {largest allowable value in a |halfword|}

And on line 2409, it says: "The values defined here are recommended
for most 32-bit computers."

I use amd64. I can deal with max_quarterword=65535 etc.

Why aren't these quarterword/halfword bounds configurable?