The wiki code:

{{wiki:dokuwiki-128.png?20}} ...

The result is or should be displayed between the horizontal lines:
(Now with a patch corresponding to https://github.com/splitbrain/dokuwiki/pull/2420/files)

The wiki code:

{{wiki:dokuwiki-128.png?128}}

• Provider/Server: 1&1
• DokuWiki version: Release 2018-04-22a “Greebo”
• PHP version: 7.2.10

https://forum.dokuwiki.org/post/63342
https://github.com/splitbrain/dokuwiki/issues/2474#issuecomment-432961717

• show document root
• phpinfo()
• php.ini
• php-errors.log
• Test-Page with Inline-PHPINFO()-Output