It should. Try to change the roaming network since probably the short access numbers (9911) are not configured correctly on the current roaming network. If you are blocked by a foreign network upon the first attempt to register retry it up to three additional times because the refusal is often caused by "roaming steering" where your home provider manipulates SS7-signaling in order to avoid your SIM to register on networks with subpar wholesale prices. However after three failed registration attempts the steering server induldges as it assumes that you are outside of coverage of your home provider's preferred roaming network.

unfortunately no,
I tried all 3 netowrks in Greece but 9911 number isn't accessible with all these 3 networks.

Then try calling your own number with 99 inserted between the mobile prefix and your individual number.
Be aware that mobile prefixes starting with 016 and 017 are 4-digit while those starting with 015 are 5-digit.

Here are some examples how the mailbox number is being created:
0163/1234567 => +49 163 99 1234567
0178/1234567 => +49 178 99 1234567
01577/1234567 0> +49 1577 99 1234567

You should also know that once you disable your mailbox there will still be a "feature" called "caller info by SMS", which will notify you by SMS about every missed call. Unfortunately if this feature answers the call it will actually be billed to the caller. With Aldi Talk will always have to pay if they try to reach you.

thanks but with this way i cannot setup the pin-password for the mailbox. this is only to hear your messages with typing the already setted-up pin-passowrd.

setup of mailbox can only be done by calling 9911.

What are the expiration terms of the Aldi sim card and the balance on it? 12 or 24 months after last recharge or last chargable call/sms?

The SIM and the credit is valid for 12 months but any consequential recharge will prolong activity and validity for 24 months. After your balance has expired, the SIM will remain active for another two months. Actually the credit doesn't really expire since this would be illegal according to German law. It will rather be removed from your balance but you can ask eplus (the entity behind ALDI Talk) to refund the amount by bank transfer.

so the simcard will actually be valid 26months after last recharge & the credit on the sim 24 months after last recharge?

yes, you are right

i think at the moment ALDITALK has the best EU roaming pakket availiable.
4.99eur. for 60mb valid for a week, thereafter you can buy a new pakket . standard rate in EU countries is 0,49/mb with 10kb billing.

At least I don't know of any cheaper rate for EU roaming. Btw if you can't reach the short access number in order to activate the roaming package from abroad (11551) as it has been the case for the mailbox number (9911) you can still do it on the Aldi Talk website or through their Aldi Talk app.