Adding user to group

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Hi,
I'm a newbe, so I hope this is the right spot for my question.
I studying Concrete5 to evaluate it for a web-site.
I added some groups. Then I added a user and assigned him to a group. Now I want that user to belong to another group, but I fail to do that.
In the dashboard I go to Members, Search Users and click the user I want to modify.
Then I the user's details and I see a "Groups", which shows nothing (I presume there should be a list of the groups the user belongs to). Under that there is a "button" "Add group". Clicking that produces an empty page, although when viewing the source (Firefox <ctrl>-u) there is code.
Is there a bug ? Or am I doing something wrong ?

# concrete5 Version
Core Version - 8.5.2
Version Installed - 8.5.2

 
koenraadl replied on at Permalink Reply
Ansering my own question :
Use Chrome/Chromium.
My standard browser is Firefox, there I have that problem. But using Chromium all is fine.