This code

Set VO2Input = New VO2Input
VO2Input.Show

is not desirable. For a start, your variable  should have a different name
to your form. But, if VO2Input is a form defined in the VBE, then it
already exists, and you don't need a new copy of it. Simply show the
original one.

Your specific error message is going to be based on the naming confusion
between your variable VO2Input and your form VO2Input, or possibly on the
fact that you have two unloads of VO2Input and they might both be being
executed.

Try removing the VO2Input variable and the "new" statement.

You also don't actually ever need to unload the form - just hide it.