roundup default 5987:ea3485c67f94
template "responsive"

view a bug, comment rows, click on "Hide"/(German) "Verbergen"
=> A red error message appears and the comment is not hidden.

Do you mean the remove button on the far right of the title line?

If so "Hide" is the wrong translation for that as it removes the 
comment from the issue, it doesn't hide it (as that implies you can 
show it later).

Also I do see a crash when using remove with the responsive tracker. It 
isn't a red error though. It's a traceback:

<type 'exceptions.KeyError'>: 'icing'

Debugging information follows

    Looking for "icing", current path:
        "page" (<roundup.cgi.engine_zopetal.RoundupPageTemplate 
instance at 0xb549996c>)

when trying to display: _generic.404.html

Could you check the error log and see if this is the error you are 
seeing? I have no idea how to fix it, so if you have an idea feel
free to add a patch.