Dear Christian,
ok, this is weird. I just tried
URLExecute["http://exampledata.wolfram.com/authentication.php"]
and get:

I suppose that's not ideal. PacletInformation gives
{"Name" -> "OAuth", "Version" -> "11.1.49", "BuildNumber" -> "",
"Qualifier" -> "", "WolframVersion" -> "11.0+", "SystemID" -> All,
"Description" -> "", "Category" -> "", "Creator" -> "",
"Publisher" -> "", "Support" -> "", "Internal" -> False,
"Location" ->
"/Applications/Mathematica 11.2.app/Contents/AddOns/Applications/OAuth",
"Context" -> {"OAuthLoader`", "OAuth`"}, "Enabled" -> True,
"Loading" -> Automatic}
and TracePrint does not finish (status us running) but gives this output:
OAuthDialogDump`Private`MultipleKeyDialog[Factual,{Key>KEY},https://www.factual.com/contact/new#free_api_access,http://www.factual.com/tos]
Here's a screenshot:

Note the black bar at the right hand side. I cannot abort, but have to kill the kernel to make it stop. Then I get:

Also keep in mind that this is not yet confirmed by any other user. In fact, Arno says that he can connect to Twitter.
On earlier versions of Mathematica all works fine. I hope this helps.
Best wishes and thank you,
Marco