Bluespice Mediawiki Visual Editor does not work

Hi, I have problems with Visual Editing. It does not show the current page in the editor.
The Source editor seems to be ok.
The error I see in the parsoid.log:

{"name":"parsoid","hostname":"blues-84853276","pid":1496,"level":40,"logType":"warn","wiki":"wiki$0","title":"Main Page","oldId":null,"reqId":null,"userAgent":"VisualEditor-MediaWiki/1.31.5","msg":"non-200 response: 403 <!DOCTYPE HTML PUBLIC \"-//IETF//DTD HTML 2.0//EN\">\n<html><head>\n<title>403 Forbidden</title>\n</head><body>\n<h1>Forbidden</h1>\n<p>You don't have permission to access this resource.</p>\n<hr>\n<address>Apache/2.4.25 (Univention) Server at mail.server.com Port 443</address>\n</body></html>\n","longMsg":"non-200 response:\n403\n<!DOCTYPE HTML PUBLIC \"-//IETF//DTD HTML 2.0//EN\">\n<html><head>\n<title>403 Forbidden</title>\n</head><body>\n<h1>Forbidden</h1>\n<p>You don't have permission to access this resource.</p>\n<hr>\n<address>Apache/2.4.25 (Univention) Server at mail.server.com Port 443</address>\n</body></html>\n","levelPath":"warn","time":"2020-02-07T15:28:06.617Z","v":0}
{"name":"lib/index.js","hostname":"blues-84853276","pid":1496,"level":60,"levelPath":"fatal/request","msg":"Config Request failure for \"https://mail.serfer.com/bluespice/api.php\": 403","time":"2020-02-07T15:28:06.627Z","v":0}

The important part being:

Forbidden
You don't have permission to access this resource.
Apache/2.4.25 (Univention) Server at mail.server.com Port 443

Any idea how to resolve it?

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