Hey Jonathan,
Hm, we unable to figure out which issue caused this, but based on what we tested, it seems like as long as it go through that ajax call, it will failed.
I've applied some temporary fix on this file
JoomlaFolder/plugins/system/sh404sef/sh404sef.php to prevent go through this method process, so it should work fine now.
Perhaps you can consult with Sh404sef developer regarding this and see whether they can figure out something we need to fix that ajax url? Because I tested in my locally, it seems work fine for me.
Keep us update If Sh404sef developer got any response of this case.