[nerbert]

This is how I solved it. You can take it or leave it.

Line 481 includes/class_wysiwygparser.php

if (preg_match('#attachment\.php\?attachmentid=(\d+)#si', $img_url, $matches) AND preg_match('#class=(\'|"|)([a-z0-9_\-\s]+)?\s*previewthumb\s*([a-z0-9_\-\s]+)?(\\1)#si', $fullurl))

There was an unescaped hyphen and unescaped dot in the regular expressions.