The licensing system is global, I think.
If that’s the case, we do not have any licensed extensions but still we confirmed in a live session with our firewall guys, that during extension upgrader runs requests to store.xwiki.com are triggered. 
I am wondering, how I could find out, which extension(s) are triggering these requests?
It’s just an extension repositories like any other, so XWiki search for available extensions there too. It does not mean you installed a paid extension.
Ah yes, okay. I interpreted Vincent’s post as if the request is triggered by having an extension from this store installed.
So, to sum up the recent discussion:
- It is expected and normal, that the extension upgrader communicates with
store.xwiki.comindependently from having paid extensions installed. - Blocking these requests was probably not the cause of the problem and the solution was not allowing those requests in our firewall.
- The solution was probably that we manually uninstalled some incompatible extensions:
- the java-flexmark stack in versions 0.64.0 (in the right order to avoid the error messages from above)
- pegdown
- the parboild java stack
- Markdown 1.0/1.1/1.2