fonsinchen opened the ticket and wrote:

If you have a file ./save/autosave/network_client.tmp you can't join a network game. Openttd will download the actual network_client.tmp to ./.openttd/save/autosave/network_client.tmp and then try to load ./save/autosave/network_client.tmp which of course instantly desyncs. This is obvious if you look at the backtrace:

FioFOpenFileSp (filename=0xaa4a18 "network_client.tmp", mode=0xb80aa1 "rb", sp=SP_FIRST_DIR, subdir=AUTOSAVE_DIR, filesize=0x0)
at /home/alve/projekte/openttd/src/fileio.cpp:345
# 1 0x00000000006dc528 in FioFOpenFile (filename=0xaa4a18 "network_client.tmp", mode=0xb80aa1 "rb", subdir=AUTOSAVE_DIR, filesize=0x0)
at /home/alve/projekte/openttd/src/fileio.cpp:380
# 2 0x00000000008906ce in SaveOrLoad (filename=0xaa4a18 "network_client.tmp", mode=0, sb=AUTOSAVE_DIR, threaded=true)
at /home/alve/projekte/openttd/src/saveload/saveload.cpp:2352
# 3 0x00000000007f0e8e in SafeSaveOrLoad (filename=0xaa4a18 "network_client.tmp", mode=0, newgm=GM_NORMAL, subdir=AUTOSAVE_DIR)
at /home/alve/projekte/openttd/src/openttd.cpp:885
# 4 0x000000000076f9b7 in ClientNetworkGameSocketHandler::NetworkPacketReceive_PACKET_SERVER_MAP_DONE_command (this=0x14af860, p=0x1762150)
at /home/alve/projekte/openttd/src/network/network_client.cpp:749
[...]

Obviously it's allowed to load things from SP_FIRST_DIR (= SP_WORKING_DIR), which it shouldn't.

Reported version: trunk
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/4271