Change #1238605 had a related patch set uploaded (by Samwilson; author: Samwilson):

[mediawiki/core@master] Special:WatchlistLabels: Use proper icon for 'add' button

https://gerrit.wikimedia.org/r/1238605

As mentioned on gerrit:

I've tried clearing the cache etc..

That's odd. I can't replicate. What is your config for $wgLocalisationCacheConf?

In T416813#11609567, @Samwilson wrote:

That's odd. I can't replicate. What is your config for $wgLocalisationCacheConf?

It's the default that DevelopmentSettings.php sets :/
$wgLocalisationCacheConf['store'] = 'array';

If you comment out $wgLocalisationCacheConf does it work? Do you have $wgCacheDirectory set?

I really need to tidy up my localsettings.. managed to fix this and its working as expected :)

Change #1238605 merged by jenkins-bot:

[mediawiki/core@master] Special:WatchlistLabels: Use proper icon for 'add' button

https://gerrit.wikimedia.org/r/1238605