collabora on nextcloud -- internal server error

[Pratched]

As I spent 1 complete day to solve this problem with already opened issues and threads on the internet without any solution, this is my last hope. Otherwise I have to pause using collabora for quite some time.
Any help or hint will be appreciated.

I have got an installation of Nextcloud 14 on a Debian 9 Stretch system with Apache2 and LetsEncrypt up an running very smoothly. As I followed the installation guides on this site combined with the offical guide here, I get into trouble every time I am trying to edit any office-file.

Nextcloud is running at domain.com and I used office.domain.com for Collabora, both of them have a separate certificate and for the docker run ...-command I used office.domain.com as well as put this address in the Admin-Settings of Nextcloud/Collabora Online.

Meaning Nextcloud is showing an "Internal Server Error" with the IP and an RequestID which I have looked for and found at data/nextcloud.log
Following is the last entry I pulled from this log-file and the RequestID definitely matches the one I got.

{"reqId":"dJUbNcnzkaXd1vKzYai8",
"level":3,
"time":"2018-12-11T21:44:13+00:00",
"remoteAddr":"_______",
"user":"____",
"app":"index",
"method":"GET",
"url":"\/index.php\/apps\/richdocuments\/index?fileId=4463&requesttoken=pZlHcIrbB6bU%2BlJ%2BSX2BvXZx2hPZ1IzbcG2xMyt10p0%3D%3AwOoEJN%2BiSOGmzWsaHgqz2R0ooFSavfWfJB3oSXEPmvo%3D",
"message":{"Exception":"Error",
	"Message":"Call to a member function xpath() on boolean",
	"Code":0,
	"Trace":[
		{"file":"\/var\/www\/nextcloud\/apps\/richdocuments\/lib\/TokenManager.php",
			"line":148,
			"function":"getUrlSrc",
			"class":"OCA\\Richdocuments\\WOPI\\Parser",
			"type":"->",
			"args":["application\/vnd.oasis.opendocument.presentation"]},
		{"file":"\/var\/www\/nextcloud\/apps\/richdocuments\/lib\/Controller\/DocumentController.php",
			"line":179,"function":"getToken",
			"class":"OCA\\Richdocuments\\TokenManager",
			"type":"->",
			"args":["*** sensitive parameters replaced ***"]},
		{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php",
			"line":166,
			"function":"index",
			"class":"OCA\\Richdocuments\\Controller\\DocumentController",
			"type":"->",
			"args":["*** sensitive parameter replaced ***"]},
		{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php",
			"line":99,
			"function":"executeController",
			"class":"OC\\AppFramework\\Http\\Dispatcher",
			"type":"->",
			"args":[{"__class__":"OCA\\Richdocuments\\Controller\\DocumentController"},
			"index"]},
		{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/App.php",
			"line":118,
			"function":"dispatch",
			"class":"OC\\AppFramework\\Http\\Dispatcher",
			"type":"->",
			"args":[{"__class__":"OCA\\Richdocuments\\Controller\\DocumentController"},"index"]},
		{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Routing\/RouteActionHandler.php",
			"line":47,
			"function":"main",
			"class":"OC\\AppFramework\\App",
			"type":"::",
			"args":["OCA\\Richdocuments\\Controller\\DocumentController",
				"index",
				{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},
				{"_route":"richdocuments.document.index"}]},
		{"function":"__invoke",
			"class":"OC\\AppFramework\\Routing\\RouteActionHandler",
			"type":"->",
			"args":[{"_route":"richdocuments.document.index"}]},
		{"file":"\/var\/www\/nextcloud\/lib\/private\/Route\/Router.php",
			"line":297,
			"function":"call_user_func",
			"args":[{"__class__":"OC\\AppFramework\\Routing\\RouteActionHandler"},
			{"_route":"richdocuments.document.index"}]},
		{"file":"\/var\/www\/nextcloud\/lib\/base.php",
			"line":987,
			"function":"match",
			"class":"OC\\Route\\Router",
			"type":"->",
			"args":["\/apps\/richdocuments\/index"]},
		{"file":"\/var\/www\/nextcloud\/index.php",
			"line":42,
			"function":"handleRequest",
			"class":"OC",
			"type":"::",
			"args":[]}],
	"File":"\/var\/www\/nextcloud\/apps\/richdocuments\/lib\/WOPI\/Parser.php",
	"Line":46,"CustomMessage":"--"},
"userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko\/20100101 Firefox\/63.0",
"version":"14.0.4.2"}