Success. Even if I can't explain how/why. I took my phone and my muscle memory "remembered" the correct pattern. At first I was happy with that, of course, but then I realized that it was the pattern that I tried a dozen times in the last 2 days. I also wrote down all patterns that I tried and it was the very first!
How come it suddenly worked? Now knowing the pattern I locked my phone again and unlo... wait. I was in. No pattern any more. it just disappeared!
I opened the settings app and searched for the security settings for "display lock". Before, I could decide between "PIN" (numbers only), "Password" (alphanumeric) and pattern. "Face unlock" and "fingerprint" were additionally available AFTER I set up a display lock.
But now I can only choose "unlock password" which is a fixed length (6 digit) number. Why do they call it "password" if it's just a PIN?!
So... why are the other options gone? Why can't I choose the pattern anymore? Why did it suddenly disappear? Questions after questions.
My system information tell me that the phone still runs with android 10 and EMUI 10, the last security patch was on "01.07.2021" (july 1st). Also "USB debugging" was disabled. There was no software update for a long time.
I am glad that I can use my phone again but I felt like the dumbest person on earth since I own a repair shop and I repair smartphones since 2010. Every time a customer tells me that he did not make any backups (what I did!) and that he did not set up any recovery option for his phone, I get a little angry with him.
And then I could not access my own phone even I set up
- huawei ID
- google "find my phone"
- "Lost android" by http://www.androidlost.com
- SMS commands (like unlocking my fckn phone!) by http://www.androidlost.com
- USB debugging with an authorization for my computer but nothing of that worked.
In conclusion: I may not be as stupid as I felt and I'll never get an answer on how that happened. But I'm very happy.
thanks for your time @alecxs