Thank you for taking care of the forum and fighting the scammers..
Actually, I started following the malicious advice as soon as I obtained the message through e-mail notification.
This way, nervous newbies like me can miss flags/trust scores etc.

User:
dj6230
https://bitcointalk.org/index.php?action=profile;u=128222

The PM from dj6230 was:
Hi,
Mycelium uses the same bip39 standard and derivation paths as Ledger so let's use the Ledger toolkit to try to locate the address holding the funds:
1. Open https://ledgertoolkit.com
2. Where it says "BIP39 Mnemonic" paste or type your 12 word seed from Mycelium
3. Scroll down and click "BIP32"
4. For client select "Coinomi, Ledger"
5. It will show ALL BTC addresses derived from your 12 word seed, try to locate the btc address hodling the funds
Reply when you have found it and i will show you how to import it again with the funds visible.

Well, my real problem begins now! It seems that I have just been tricked and robbed.
A user of this forum advised me by PM to use "ledgertoolkit.com" to inspect stuff. In the stress, I neglected security and typed my 12 words there. Within few minutes, transactions started to appear on blockchain.com, forwarding and splitting the BTC.

Hi all,
I will be very glad for any help in my stressful situation.. thanks in advance.

I had a single wallet backed-up with 12 words, running on two phones in parallel. Yesterday I decided to make a fresh start with new 12 words, so I took one of the phones, reinstalled Mycelium, created a new account, generated new 12 words backup, set up PIN, and tapped  "Cold Storage" > "Masterseed". I entered the old 12 words and the app showed a message like "successful spending" followed with the correct amount.

Since that (ca. 20 hours from now), both the phones show 0 BTC. The old-account phone shows "2 private keys" which, I think, is less than before. Using transactions listed by the old-account phone I located my money on "blockchain.com/btc/address/XYZ" - the amount is still shown there, no transactions happened during the critical time. The new-account shows "3 private keys" which is the default, I think.

Please, help. Yours,
Vojta