This a problem that most exchanges have with regard to fees on trades and what EWallets have when they charge fees on withdrawals/spending. The problem is that "math is hard".
The UI for most of these do nothing to make it so that user isn't forced to know basic math in order to spend the remaining balance.
I've never used FlexCoin so I don't know how they handle their transactions, but here's a typical scenario:
If the fee is 1% and I have 0.80000012 BTC, then the calculation for the fee is 0.008000001. But that is sub-satoshi, so it rounds up and treats the fee as 0.00800001, leaving 0.79200011 as the amount that can be withdrawn.
But my own calculation after manually subtracting the fee I come up with 0.792000119 BTC of funds. If I try to enter that amount, the system knows that 0.792000119 is a number greater than 0.79200011 and thus I am trying to withdraw more than my available balance.
If the service were to allow that to occur, what would happen is bots would be written to take advantage of this, earning 1 satoshi at a time, millions of times.
Something like this rounding-related issue is likely what is happening here.
There are a number of methods of addressing this. For instance, whenever attempting to withdraw more than your balance, the service could calculate and display the maximum amount after fees that you can withdraw. You click OK and the transaction completes. Problem solved.