I've decided to start a separate topic, although this may also be related to the previous one ("Spoiler-Button instantly disappears (although it is shown for a spöit second)"). The difference is that I do not see the button while viewing a page normally at all, only text "Show spoiler".
The following two points are new in my case:
- There is a fully functional button in edit preview
touch LocalSettings.php or any other changes to the file fixes it until I edit any page in the wiki and save the changes
I use MySQL+PHP as shipped with Ubuntu 14.04.2 with Mediawiki 1.25.2 and Spoilers extension v.1.3. I've been able to reproduce the steps above on Bitnami Mediawiki Virtual Machine in Virtualbox. We might use it as a reference environment, too.
The problem is repoducible in both Firefox 40 and Google Chrome 44.0.2403.155.