Found out some more (and some new "funny" stuff occured)
Apparently AU is using new servers (not for all phones tho..), which probably allow smaller mail sizes (whyever).
As to the game problem.. after asking in two au shops and even calling the center (who said I should send it in for repair), I actually did send it for repair... and it came back after 9 days (they said 6-7 but whatever). Went to the shop to get it back, thought it shouldn't take more than 10 minutes.. yet again I was a dreamer. The repair center answered that they couldn't find any problem but re-installed the software or so. Now when the staff put my sd card back, the game showed the same problem like before. Thus we concluded it must be the sd card, as it worked with the one AU used. Which means it wasn't even necessary to send it to the repair center if someone had had the idea (aka. knowledge) it could be the sd card..
Now here comes the "funny" part, I said "ok, than you just have the replace the broken card". Answer: "Yes, but actually we don't have such a service..."
Me: "........

"
She was obviously embarrassed about this (especially knowing all the trouble I've had) and called the allmighty "center" to check and apparently they said sth like the sd card is innitially not included, but just offered by them as an extra... (my phone doesn't have sim-card, works with sd cards)
I.E. what AU basically says is "buy our phone, we will even give you an 8gb sd card (which you will definitely need anyway) with it, don't guarantee that it works tho".. after the staff girl spoke with a colleague, she said she'll give me their (used) sd card, but as a secret as it's actually not allowed.. really, Japan must generally have the best service in the world (obviously only as long as it's by the rule book), but sometimes...
Then she was reprogramming my phone for the number and mail address, but there was a problem with calls, it didn't work. Finally she could solve it after another call to the center (but it took another 15 min. or so).
Another 2 hours spent there (must be about 10h by now).
When I had my old phone I never had any trouble with au for 5 years, but maybe the smart phone business is too much for them!? (maybe the competitors aren't better tho, I don't know)