@donw, a couple of things you could do:

1. Install Report All Errors (https://www.zen-cart.com/downloads.php?do=file&id=1792) and then set Configuration->Logging->Report All Errors Admin to IgnoreDups and Backtrace on Notices to true). That will help me to determine some of what's going on.
2. Enable Edit Orders' debug (Configuration->Edit Orders->Debug Action Level) to 1.

Create a test order, split it and then attempt to edit it. I'll be 'looking for' two types of logs:

1. /logs/myDEBUG-adm-*.log ... files that were created during (a) the initial EO 'view' of the order and (b) the one that includes the fatal error.
2. /logs/edit_orders/debug_edit_orders_xxx.log, where xxx is the split order being edited.

I'll PM you my direct email address, as those logs (especially the edit_orders one) can get pretty big.