I get a Prism error when I try to export for a backup:

XML Parsing Error: undefined entity
Location: chrome://mozapps/content/downloads/downloads.xul
Line Number 55, Column 5:
<key id="key_findDownload2"
The caret is between < & k

The backup file is created but with size 0 or the "part" file exists without its completed download.

Export from local folders also gives an error but the file is completed - I haven't experimented further with this.

I get a pop-up telling me to try saving to a different location but there is plenty of space and I have the correct permissions. Export from a normal browser (Firefox) and from reloaded Beta1 is OK.

Not sure if it's relevant but the I have tried both US and GB as the locales.