233: Malformed textbox

You started a line with a left square bracket [, followed by one of the characters _ = -, followed by a colon :. This pattern is recognized as the start of a textbox with placeholder, a kind of form field. But a right square bracket ] to mark the end of the textbox is missing in your case.

Solution

Make sure you end the textbox with a right square bracket ] as in the following example:

[_:yyyy-mm-dd] your date of birth

Mind that there can be no empty line in the textbox markup.

If you did not mean to have a textbox and just wanted to print a literal left square bracket at the start of a line, you can prepend it with a backslash like this: \[