WikiText not decoded to HTML on Display

Hi all,
just setup RT with Articles and CF type of “wikitext”.

Unfortunately the value of this CF is not decoded to HTML and shown as is
in DB.
Example:
Database entry: "=== General rules === Always create OS filesystem on
separate disks."
is shown without heading formatting and with all the ‘=’ characters.

Went through the documentation, but wasn’t able to find anything which
would help me to solve this issue.

RT version: 4.2.10, Text::Wikiformat version 0.81 installed from CPAN.

Is there anything I should check? Even debug logging doesn’t show any issue.

Thank you in advance.
Peter