Unknown key “n/a” in older file

Glyphs 3 cannot open a file that Glyphs 2 can. The message I get is

Unknown key “n/a” on line: 3415

The oldest version of the file must have been made in the time of version 1200. So it may be a known relic of the past.

In the text version I find:

{
color = 6;
glyphname = seven;
lastChange = "2021-01-25 21:32:38 +0000";
layers = (
{
background = {
paths = (
{
closed = 1;
nodes = (
"0 0 n/a",
"226 0 LINE",
"277 275 OFFCURVE",
"409 499 OFFCURVE",
"570 704 CURVE",
"570 712 LINE",
"110 712 LINE",
"110 700 LINE",

and so on…

I think it is clear which one is line 3415.

There are about 60 lines with 0 0 n/a in various places. It appears to have to do with backgrounds, but in the UI (Glyphs 2) I cannot see if there is something wrong. When I remove the backgrounds of all glyphs with Glyphs 2 the problem goes away.
I am not sure if I can send the file.

You should be able to remove all those lines manually.