List of steps to reproduce (step by step, including full links if applicable):

• Open two browsers, with two admin accounts
• Open the https://nl.wikipedia.org/wiki/Speciaal:Blokkeren/<some_username_with_a_space> of the same user in both browsers
• with browser 1, apply a block
• With browser 2, apply a block
• This gives the warning that is wrongly formatted

What happens?:
The displayed warning contains a formatting error. In this case, the username was "Mark K***e" (removed explicit string to reduce findability. This gives this html:

<a class="external text" href="https://nl.wikipedia.org/w/index.php?title=Speciaal:Ipblocklist&amp;action=unblock&amp;ip=Mark">K***e deblokkeer</a>

Screen shot:

What should have happened instead?:
The space in the username should have been converted to an underscore. The unblock link consequently also does not work.

I would expect this html:

<a class="external text" href="https://nl.wikipedia.org/w/index.php?title=Speciaal:Ipblocklist&amp;action=unblock&amp;ip=Mark_K***e">deblokkeer</a>

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc: n/a