https://forum.dokuwiki.org/post/55523
You should update “Gallery Plugin” before the DokuWiki update!
If you didnt, you need to disable the plugin:
To get into the adminmenu, disable javascript
or
edit “plugins.local.php” and insert “$plugins['gallery'] = 0;”
or
remove the folder “lib/plugins/gallery” and reinstall the plugin later
https://github.com/splitbrain/dokuwiki/issues/2333
The error might occur when your PHP version is 5.5 or lower.
You need atleast 5.6 and soon 7