Not, it was not in the list. I tried it again on the console and got:
root@DietPi:~# pihole -b -wild 51yes.com
[i] Adding -wild to blacklist...
[i] -wild does not exist in whitelist, no need to remove!
[i] Adding 51yes.com to blacklist...
[i] 51yes.com does not exist in whitelist, no need to remove!
The funny thing is: Now I have two entries -wild in Exact blocking and can't get rid of them. Whenever I delete them and reload that page they are back
As you already found, you need --wild for wildcard entries.
However, wildcard and regex entries are not the same as "wildcard" entries will be added to the regex.list prepended with (^|\.) to ensure to get regex that are compatible with what we previously (i.e., pre-v4.0) called wildcards.
For adding regex entries, you need to run
$ pihole -b --regex "51yes.com"
[i] Adding 51yes.com to regex list...
[i] 51yes.com does not exist in whitelist, no need to remove!
This means that this line had no trailing newline character (\n). If you haven't edited the file manually, it is unclear how this could come about. I tried a few things you mentioned here but still wasn't able to trigger such a thing.
This simply means that your web session has expired. If you would have gone to any other page (or reloaded the page by pressing F5), you'd have seen that you need to re-login.
@DL6ER: Thanks for your detailed answer - see my notes below!
Okay, but I had added, e.g., 2o7.net or 302br.net before in the admin console by clicking "Add (wildcard)" and they had been added as regexes as mentioned in my first post. So I still don't understand why it didn't work with 51yes.com.
This exactly happened again when I executed above command: 51yes.com was attached to (^|\.)2o7\.net$: it looked like(^|\.)2o7\.net$51yes.com. After deleting this modified entry, it finally worked. Then I removed (^|\.)51yes\.com$ again, entered 51yes.com and clicked "Add (wildcard)" - and surprise, surprise: Now it was accepted! I'm not sure why this inconsistent behaviour happened. Could it be that clicking "Add (wildcard)" does not (always) add a trailing newline character?