Hi John,
the issue it's connected to a crash bug that we have on the max number of lines that a document can have.
(To repeat it, just try to create an invoice or a sales order, with more than 201 lines)
I've filled a bug report and escalated your issue.
The only available workaround, to not get stuck in a situation where it's not possible to create for example, a delivery,
it's to manually keep the number of lines in a document, below 200.