Thank you so much, everybody.
And once more, sorry for the mess!

In my naivety, I was hoping to avoid the procedure by just creating another account. Obviously not my smartest move :) I'm sorry for the mess!

@hashar
I noticed that the problem was the 2FA for AndreG-P. It is broken for this account. Unfortunately, after I reset the password for AndreG-P, it is still asking for the 2FA which does not work. So I cannot change the e-mail for AndreG-P because I cannot enter the account settings without 2FA.

@hashar
Thank you very much. That explains the problem a little bit. However, I am still not able to solve it.

@Aklapper In https://gerrit.wikimedia.org/r/login I enter the same credentials that I am using at https://wikitech.wikimedia.org/ to login.
But on https://gerrit.wikimedia.org/r/login, I got an "Authentication failed."

@Physikerwelt @alexhollender This seems to be a valid option for me. However, somebody should confirm that an additional pop-up library is acceptable. Otherwise, we run into a dead-end again.

@Physikerwelt A description of the special page Special:MathWikibase.

@Physikerwelt According to this post "omicron not working in latex" from 2015:

@alexhollender, I'm sorry for my late response. I have trouble setting up my new system to the point when I created the popups. So let me explain the concept a bit more in detail. Clicking on the popup will lead to a special page with all information, e.g. for the mass-energy equivalence it is this page: Special:MathWikibase&qid=Q35875

@alexhollender yes the picture is up to date.
I updated the picture in the issue description also.

@thiemowmde Thank you. The problem is, I thought I did that already. I load the modules in the special page:
https://github.com/wikimedia/mediawiki-extensions-Math/blob/master/src/SpecialMathWikibase.php#L64

		$output->addModules( [ 'ext.math.wikibase.scripts' ] );

Thank you for the hint. Patch set 5 should include all necessary changes to close this task: https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Math/+/527556/

@Physikerwelt an autocompletion on wiki data IDs? I would understand if we allow textual inputs here and then activate auto-completion based on the words. Such as "mass-energy" and the first suggestion would be "equivalence". But I cannot imagine an auto-completion just based on the IDs. The only use case I can imagine is that the auto-completion only shows IDs that actually contain mathematical expressions. But I have no clue how to implement that now.

Hi everybody, I'm close to a prototype now. I created an extra API endpoint in the math extension and send requests to this endpoint if the user hovers over annotated math in the Popups extension.

Another update.
After I requested some help from the #wikimedia-tech IRC channel it seems to be better not to use special pages to fetch the data from wikidata but extend the popups extension so that it performs the fetch from wikidata by its own. The problem is that special pages do not contain "text" in a sense that the TextExtracts extension can extract this text. The mods in the IRC channel mentioned changing the TextExtracts extension would be a major re-engineering effort. Since the popups extension handles page previews and reference previews differently already, it would make sense to just add a special case for wikidata previews to the extensions. It's questionable if the popups extension should handle math specific content, but it seems to be the best option we have.

@thiemowmde do have an idea why we cannot fetch text via TextExtracts from special pages? I want to visualize as special page in a popup but the result from mw.Api.get does not return text from special pages.

I was able to detect the problems that avoid visualizing special pages.
The type of special pages is previewTypes.TYPE_GENERIC but the code currently only supports the cases in line 135 in src/ui/renderer.js.

I needed to create a Title object from a special page. The popup extension I'm using use the javascript API. So I need an object of mw.Title (https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Title) from a special page. However, I'm not able to generate the right Title... Does somebody know how to do it?

@Aklapper Yes, it's not really a fix but kind of an extension we (@Physikerwelt) and I are planning to accomplish.