The following error Failure when converting editor format to storage format. can be reproduced with Confluence 4.3.2 as follows:

1. Send an email to Confluence (any space that can receive emails).
2. View this email in Confluence and copy the ID (e.g. 20152482 for email at http://wiki/display/TS/mail/20152482)
3. Create or edit a page and insert a link to this email by inserting [$IDOFMAIL] (e.g. [$20152482], as described in https://confluence.atlassian.com/display/CONF35/Linking+to+Mail).
4. Try to save the page - the error will show up. Any page containing such links from older versions will display properly, but will fail at the next edit.