I have same issue when I do `ls /mnt/webdav/lost+found` with mounting webdav.

Here is a patch to fix segfault.
backup(lost+found) node would be different from webdav node.

--- a/src/cache.c
+++ b/src/cache.c
@@ -1022,7 +1022,7 @@ dav_lookup(dav_node **nodep, dav_node *parent, const char *name, uid_t uid)

         if (is_open(*nodep))
             attr_from_cache_file(*nodep);
-        else if (!is_dirty(*nodep)) {
+        else if (!is_dirty(*nodep) && !is_backup(*nodep)) {
             int ret;

             dav_props *props = NULL;

This bug also arises, when you have files in lost+found and try to acess them:

cd lost+found/
[marc@localhost lost+found]$ ls
ls: cannot access 'dir-lost+found-xhbokx': Software caused connection abort
ls: cannot access 'dir-lost+found-rLn3pO': Transport endpoint is not connected
....

Reproduce

1. Setup an Nginx server with dav module enabled.
2. On another server, mount the resource serving by nginx.
3. In the mounted directory, create a file contains '[', e.g.

 echo test | tee 'mounted/[test]'

4. Try to cat the content, dmesg will show a crash something like

[145885.623406] mount.davfs[82429]: segfault at 18 ip 000055ce8851c8ec sp 00007ffce9ccd540 error 4 in mount.davfs[55ce88515000+15000] likely on CPU 0 (core 0, socket 0)
[145885.623416] Code: 00 48 c7 44 24 08 00 00 00 00 48 8b 7b 20 48 8d 74 24 08 67 e8 b5 b6 00 00 85 c0 0f 85 69 ff ff ff 48 8b 7c 24 08 49 8b 5d 00 <48> 8b 47 18 48 39 43 48 74 54 48 89 43 48 48 8b 43 30 48 85 c0 74
[145885.623494] audit: type=1701 audit(1675728425.394:461): auid=972 uid=65534 gid=90 ses=5 pid=82429 comm="mount.davfs" exe="/usr/bin/mount.davfs" sig=11 res=1

The Nginx config (block)

location /dav/ {
    alias /tmp/;
    client_body_temp_path /tmp/nginx/;
    charset utf-8;
    dav_access user:rw group:rw all:rw;
    dav_methods PUT DELETE MKCOL COPY MOVE;
    dav_ext_methods PROPFIND OPTIONS;
    create_full_put_path on;
    autoindex on;
}

Append

I tried using this simple script as webdav server: https://gist.github.com/mickael-kerjean/f2f034bdad5e077edcbfdff649d52d68

And the above steps won't make any crashes, the file content can be normally shown.

Server OS is Ubuntu 22.04 LTS.
Client (who runs mount.davfs) OS is Arch, davfs2 installed from AUR (https://aur.archlinux.org/packages/davfs2)