FA launches then immediately crashes with the following error:

When downloading an old replay for an older version of the game, FA downloads the necessary files to make the game function properly, the game launches and then crashes with this error message; can anyone make heads or tails of it?

java.lang.RuntimeException: Forged Alliance Crashed with exit code -2147483645. See game_1767521.log for more information
at com.faforever.client.game.GameService.lambda$spawnTerminationListener$45(GameService.java:719)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:832)

The error message that you posted is from the client. It is the client's way of saying "the game crashed" but the client doesn't know why.

The interesting details are going to be in the log file "game_1767521.log"

Seems this is what caused the fatal error:

warning: ...mdata\faforever\gamedata\lua.nx2\lua\skins\skins.lua(279):
attempt to call field 'assimilate' (a nil value)
warning: stack traceback:
warning: ...mdata\faforever\gamedata\lua.nx2\lua\skins\skins.lua(279):
in main chunk
warning: [C]: in function 'doscript'
warning: [C]: in function 'pcall'
warning: ...alliance\gamedata\mohodata.scd\lua\system\import.lua(48):
in function 'import'
warning: ...alliance\gamedata\mohodata.scd\lua\system\import.lua(37):
in function 'import'
warning: ...ramdata\faforever\gamedata\lua.nx2\lua\ui\uiutil.lua(404):
in function 'GetCursor'
warning: ...ramdata\faforever\gamedata\lua.nx2\lua\ui\uiutil.lua(414):
in function 'CreateCursor'
warning: ...r forged alliance\gamedata\lua.scd\lua\ui\uimain.lua(24):
in function <...r forged alliance\gamedata\lua.scd\lua\ui\uimain.lua:20>
warning: Error running '/lua/ui/uimain.lua:SetupUI': ...alliance\gamedata\mohodata.scd\lua\system\import.lua(37):
Error importing '/lua/skins/skins.lua'
stack traceback:
[C]: in function 'error'
...alliance\gamedata\mohodata.scd\lua\system\import.lua(52):
in function 'import'
...alliance\gamedata\mohodata.scd\lua\system\import.lua(37):
in function 'import'
...ramdata\faforever\gamedata\lua.nx2\lua\ui\uiutil.lua(404):
in function `GetCursor'
...ramdata\faforever\gamedata\lua.nx2\lua\ui\uiutil.lua(414):
in function 'CreateCursor'
...r forged alliance\gamedata\lua.scd\lua\ui\uimain.lua(24):
in function <...r forged alliance\gamedata\lua.scd\lua\ui\uimain.lua:20>

Can anyone make heads or tails of <this>?

What mods do you have enabled?

Absolutely none; the game is bone-stock. However, the replay I'm attempting to load is from an ancient version; over 7 years old. (I'm not sure if we had mods enabled then, though...)