...to be able to test zmodem transfer correctly, we also need to attach from the first tab to the second one:

screen -rx zmodem

Ok, we have a patch:

% cat ~/gnu/zmodem.patch
diff --git a/src/process.c b/src/process.c
index 129e2dc..cbd33f2 100644
--- a/src/process.c
+++ b/src/process.c
@@ -1190,7 +1190,7 @@ static void DoCommandZmodem(struct action *act)
                int i;
                for (i = 0; i < 4; i++)
                        if (!strcmp(zmodes[i], *args))
-                               return;
+                               break;
                if (i == 4 && !strcmp(*args, "on"))
                        i = 1;
                if (i == 4) {
diff --git a/src/window.c b/src/window.c
index 9f5e2d4..042d152 100644
--- a/src/window.c
+++ b/src/window.c
@@ -1812,7 +1812,6 @@ static void zmodem_found(Window *p, int send, char *bp, size_t len)

 void zmodem_abort(Window *p, Display *d)
 {
-       Display *olddisplay = display;
        Layer *oldflayer = flayer;
        if (p) {
                if (p->w_savelayer && p->w_savelayer->l_next) {
@@ -1831,7 +1830,6 @@ void zmodem_abort(Window *p, Display *d)
                D_readev.condpos = D_readev.condneg = NULL;
                Activate(D_fore ? D_fore->w_norefresh : 0);
        }
-       display = olddisplay;
        flayer = oldflayer;
 }

Let's test it... git master branch, Kubuntu:

% cat /etc/issue
Ubuntu 22.04.4 LTS

% dpkg -s konsole | grep '^Version:'
Version: 4:21.12.3-0ubuntu1

% git branch
* master

% git pull
Enter passphrase for key '/home/alex/.ssh/id_rsa.screen':
Already up to date.

% git rev-parse HEAD
fcfa6d2da0735d27d322ff22147534ecfb4d7302

% git apply ~/gnu/zmodem.patch

% echo $?
0

% ./configure
% make
% sudo make install

% screen -v
Screen version 4.99.0 (build on 2024-07-10 13:07:15)

I have 3 tabs in Konsole. First one I'm going to use for the first screen session 'source', second for the second one 'zmodem' and last one for the "screen -ls", i.e. for some outside checks (there is no screen session there).

Start first screen session:

[3] % screen -ls
No Sockets found in /home/alex/.screen.
[1] % screen -S source
[3] % screen -ls
There is a screen on:
        13271.source    (Attached)
1 Socket in /home/alex/.screen.

Start second screen session and enable zmodem mode there.
Check file name (example test name... let's say "passwd")

[2] % screen -S zmodem
[2] C-a :zmodem pass

# I get message 'zmodem mode is pass'

[3] % screen -ls
There are screens on:
        13344.source    (Attached)
        13393.zmodem    (Attached)
2 Sockets in /home/alex/.screen.

[2] % ls -la passwd
ls: cannot access 'passwd': No such file or directory

Send some test file from the first screen session 'source' to the second one 'zmodem' by usging 'sz' command:

[1] C-a : !!! sz /etc/passwd
Sending: passwd
Bytes Sent:   2717   BPS:511578

Transfer complete
% echo $?
0

Second session. Check the file again.
Third tab: Check the state of both screen sessions:

[2] % ls -la passwd
-rw-r--r-- 1 alex alex 2717 Sep 28  2023 passwd
[3] % screen -ls
There are screens on:
        14102.source    (Attached)
        14118.zmodem    (Attached)
2 Sockets in /home/alex/.screen.

Both screens are alive. No dead sessions. No crashes. File is there...

git version even more worse, it cannot set zmodem mode.
see process.c:1193, should be break, not return.
and, with fixed this but still crash.


Program received signal SIGSEGV, Segmentation fault.
zmodem_parse (p=0x5636acc36770, bp=0x7ffe54f5df21 "**\030B0800000000022d\r\212d\r\212lo1.c\030@56 14641454267 100644 0 1 56\030@\030k\361F\274\067\021\033[00m$ ", len=20) at window.c:1719
1719                                            D_blocked = 0;
(gdb) bt
#0  zmodem_parse (p=0x5636acc36770,
    bp=0x7ffe54f5df21 "**\030B0800000000022d\r\212d\r\212lo1.c\030@56 14641454267 100644 0 1 56\030@\030k\361F\274\067\021\033[00m$ ", len=20) at window.c:1719
#1  0x00005636abfb7b71 in win_readev_fn (event=0x5636acc36848, data=0x5636acc36770) at window.c:1536
#2  0x00005636abfa7d22 in sched () at sched.c:200
#3  0x00005636abf5534d in main (argc=0, argv=0x7ffe54f60280) at screen.c:1102

15 yrs later, screen still crash...

ubuntu 22.04 screen 4.09.00

Program received signal SIGSEGV, Segmentation fault.
0x0000557986023a6e in zmodem_parse (len=<optimized out>, bp=<optimized out>, p=<optimized out>) at ../window.c:2177
2177    ../window.c: Directory not empty.
(gdb) bt
#0  0x0000557986023a6e in zmodem_parse (len=<optimized out>, bp=<optimized out>, p=<optimized out>) at ../window.c:2177
#1  win_readev_fn (ev=<optimized out>, data=0x557987fc2f80 "") at ../window.c:1947
#2  0x0000557986007a67 in sched () at ../sched.c:222
#3  main (ac=<optimized out>, av=<optimized out>) at ../screen.c:1486


I've debugged this. When using zmodem pass mode, a overlay page "Zmodem active" showed by InitOverlayPage, which set w_layer.l_cvlist to NULL causing global display NULL. In zmodem_abort, through ExitOverlayPage is called for restoring canvas but will take affect in next win_readev_fn call. So nearly after zmodem_abort, display is accessed and oops.

Can you please post a backtrace from the original crash? The display store/restore might need to happen somewhere else etc. The backtrace might give a better understand of the issue.