I asked 48 support about this and the answer is that once you have used up your monthly membership EU roaming data allowance (7GB/9GB for the €9.99-20GB/€14.99-40GB memberships respectively) then you cannot get more until a new membership starts and you get a new EU roaming data allowance.
I guess that if it's an emergency you could contact 48 support and ask if they can activate a new membership "early" - i.e. before your current membership runs its full monthly course and expires normally.
Hope this helps.
Thanks dalyer. What also works in an emergency is to buy an addon of 1GB for 3 EUR. It won't get you far, but at least you'd have some data on the road.
It works, just tried it the other day in Austria.
Thanks @bozicb - that's interesting and not what 48 support told me unfortunately!
So I take it that you had used up your 7/9GB of EU roaming data, maybe had some "non roaming" data left but until you bought the data add-on you had no data access while EU roaming?
Useful to know that the add-on gives you EU roaming access again if you've used up your normal EU roaming data allowance.
Thanks for that - it might help others in the same situation.