I have exactly the same issue - if a browser has logged into the site previously it works ok, but if a new browser or user is used i get the cookies are blocked etc. Message.

Any idea where to look / how to solve?

Many thanks

In the end I contacted my hosting company who resolved the issue.  It appears that somehow I'd ended up with 2 paths referenced (in php.ini) for the same add-in (ImageMagick) which was causing a conflict.

All is now good!

I am posting this, just in case anyone else has this problem.

In this recent incident it was a case of self inflicted wounding, but such a simple error.

In an HTML page menu, I had a link to the piwigo gallery

This url works fine
http://www.brandontownbowlingclub.co.uk … /index.php

This url works, but you cannot login and get the cookie error problem
http://www.brandontownbowlingclub.co.uk … /index.php

See if you can spot the difference?

The only difference is the 2 forward slashes // instead of 1.

http://www.brandontownbowlingclub.co.uk … /index.php
                                                                  --

What puzzled me, and still does, if the URL is wrong why does the page load anyway?

Hope this helps someone.