Hi mate,
As Cove said, it is an issue that has never been fixed.
Please bear this in mind and do not accept all friendly challenges at once.. You are going to have to manually accept each one. It will start taking 100 credits for every other friendly are scheduled and waiting for a fixture date.
For example:
If you had 10 scheduled friendlies waiting for a fixture date and you just accepted lets say '2' more friendlies, instead of it being 200 credits cost with the training, the bug will it make cost you 1,200 credits. This will only be the case if you had accepted both friendlies together and not one by one.
If you have lost out on a lot of credits, please let me know approx how much and I can forward this on to Junior in which he will hopefully compensate you.