ECC+ calculation cannot be solved after using browser 'back' button

After entering a wrong result of the calculation, it says something like 'please do it again'. That's fine! Unfortunatelly, the form is cleared (in my case: the contact form of AlfContact). To avoid typing in everthing again, I use the 'back' button of the browser - and all fields are filled in again

Now I see the same formula with the same numbers again - this is the effect of the back button. But the calculation cannot be solved anymore, even the correct result leads to a 'please do it again'. I have to type in everything again

I use:
Joomla! 2.5.6
AlfContact 2.0.3
ECC+ 2.5-3