continuing runtime error -- Can't locate object method, "single" via package "Vend::Table::GDBM"

Partial temporary solution found:

It appears that the error is generated when the multi-page checkout is 
selected, or if the checkout page is selected without anything in the cart.

The solution that has helped is to disable multi-page checkout. At 
least, we can get some orders processed this way.

If anyone has some further suggestions, it is appreciated.