This happened to me during the composition of a PM, and then once again in testing the regular forum posting:

I pasted a URL address into the 'Compose Message' pane as part of my message ... then I finished the message ... then I clicked 'Preview' where I checked to see if the message was as I wished it to be. As I have done a million times on the Old Board Version I clicked on the URL in the preview to check that is was a good link, then I hit the back button (as I have done a million times on the previous version) to go back to the Preview/Compose window ... and GOODBYE MESSAGE. Lost the whole thing, and quite unrecoverable.

This did not happen on the old phpBB version . . . it was always possible to check the link and then click back into the message to complete and send it.

So please be careful. If you check a URL link in 'Preview' mode, do yourself a favour and copy the entire message first. This will ensure that you can at least start again in a new 'Compose' window with your saved post intact.
This is especially annoying when it is a long and/or important post.
Not sure if there is an easy fix to this ( I hope so ), but if not then at least you can accomplish the second-best option by using the copy&paste feature.

Cheers ...