Unable to attach switch network profile to a new site

[pielunswe]

Hi!
Trying to add a switch-network profile to a newly created site, like many times before. Pushing "sites", mark the new site checkbox, and Save.

But now page is just "flashing" when pushing Save button, and nothing happens. It does not get marked in the checkbox, and its the same amount of sites as before (2106), have tried Firefox, Edge, Chrome and Opera. Have also rebooted the server. It's an XL DN2, DNA version 2.3.5.6, all services are green.

Any suggestions? 
Thanks in advance.

Blue is just masking, not related. 

[Torbjørn]

If you open the browser console(In Chrome it's F12 and select "Console"), do you get any notable output when doing it? In any case that's not normal behaviour and I would probably open a TAC case for this.