SPAM filter of the magnolia forum (https://forum.magnolia-cms.com) takes links made with the link icon as SPAM ...
Example

[url=https://documentation.magnolia-cms.com]Documentation[/url]

URLs are only allowed when pasting directly

Remove the link function from the editor menu or reconfigure the spam filter.

B.t.w.: The error message is "ugly" (general tomcat error 500 page); this would be fine in "real" SPAM attempts but is not nice in the current situation.