[CONFSERVER-28871] Failure when converting editor format to storage format with links to emails on page

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

Send an email to Confluence (any space that can receive emails).
View this email in Confluence and copy the ID (e.g. 20152482 for email at http://wiki/display/TS/mail/20152482)
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).
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.

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