Ummm. OK, no, I couldn't reproduce the problem in 5.4 I didn't get that far.
To make a long story short (after my initial loftier efforts) I simply created a basic custom list with *no* changes to the list – no columns added, etc. I can create items with that basic list. Then I do the basic dance of "replacing" the default webpart with the qListForm webpart, and when I try to save, I get the following:
