Hello,
The link to a discussion in the Activity Stream (within a group) does not work - here you end up instead in the discussion on the home page, as in the group itself (User X and 5 others have joined the group XXX).
XXX group is linked to the homepage.
Likewise, the video: User XXX and 11 others like this video.

Why it does not work all that? Where are the error here?

HI, Laura.

Please, post direct URL where I could see that issue.

Hello Michal,
you must login to see the activity stream (use User Details please as admin).

Hi, Laura.

Sure I logged in... but most of your activity stream is "user A ist jetzt mit user B befreundet ".
One post about profile cover change. And one about photo upload.

Could you post a printscreen of post you have issue with?
Or how long need I scroll down friend notices to see post you mentioned.

For example "Laura gefällt Heidi's Profil"

Please look...

Now this problem in "like group": {multiple}{actors}{/multiple}{single}{actor}{/single}

Hi, Laura.

Ok, now I know what's wrong.
Issue is caused by your language override: i.imgur.com/J6lUT7j.png
Please, note that if you create overrides for texts that contain _QQQ_ - you need to remove them in override.
_QQQ_ are used in ini file and they are equivalent of ". In ini file " is used for start and end value so if we want to have URL inside we need to use _QQQ_. When using Joomla! language override _QQQ_ are spared. joomla! converts " to _QQQ_ automatically so user don't even see that.

So you need to review your language overrides and remove _QQQ_ in them - just like I did for one of yout text (see printscreen).

As for second issue - it's related with outdated language file. We don't use that string any more. You need to:
- edit your language file and remove those strings
- look for language package that fit your current JomSocial version
- or ask other JomSocial German users to share updated version with you (you may post in our Community i German language group).