H5P in Moodle mobile app, problems

The goal is to use H5P activities in Moodle via the Moodle mobile app. As far as I know, this is possible with Moodle >= 3.5 and current H5P versions. 

The platform in question runs Moodle 3.6.4+ and the most recent H5P plugin and recent module updates. The Moodle mobile app does try to open / embed the H5P activity inline, as opposed to before when it went right to the browser. But after a while of loading, there appears a message "Website is not available" with the full embed-url of the activity like /mod/hvp/embed.php?id=336&user_id=57&secret=4w98w94...

Any idea what could cause this? 

"embed" is allowed in the moodle settings.. 

Server-Logs say: 

2019/12/10 15:10:40 [error] 16476#16476: *18461 FastCGI sent in stderr: "PHP message: Default exception handler: Notwendiger Parameter "token" fehlt Debug: 
Error code: missingparam
* line 494 of /lib/setuplib.php: moodle_exception thrown
* line 566 of /lib/moodlelib.php: call to print_error()
* line 45 of /webservice/pluginfile.php: call to required_param()" while reading response header from upstream, client: 89.0.120.xxxx, server: xxx.xxx-akademie.de, request: "GET /webservice/pluginfile.php/483/mod_page/content/10/Uhu-muc.jpg?forcedownload=1 HTTP/2.0", upstream: "fastcgi://unix:/var/run/php-fpm-www.sock:", host: "xxx.xxx-akademie.de"
2019/12/10 15:10:40 [error] 16476#16476: *18461 FastCGI sent in stderr: "PHP message: Default exception handler: Notwendiger Parameter "token" fehlt