Its not possible to update to PHP7.1 on any Debian 8 or 9 – unless you compile the whole thing from source which will conflict with the system packages.
Downgrade the plugin to the latest 3.x Version and make sure it is not accidentally overwritten by updates.
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I had a setting added that caused much trouble recently ( See here https://ww.wp.xz.cn/support/topic/wordpressmu-problem-with-edit-links/), perhaps that issue is related to this.
I will check this when new package updates are available next time.
Issue can be closed, it turned out the problems were caused by this setting in wp-config.php:
define(‘CUSTOM_TAGS’, true );
It also broke saving HTML tags in Classic Editor.
