I'm currently using SME 5.12 and was keen to upgrade to 5.5 or even better to 5.6.
The install went fine the updates applied ok.
The only immediate problem I found was when using webmail (with horde and imp) is after composing a message and clicking the send but in most browsers nothing happened or a basic error message of broken link or in IE A Null URL???
Checking the logs a nothing is showing at all even that squid said there is a problem but nothing.
After long research on the net I found an answer in another message board where
Parag Godkar had the answer
All you have todo is:
Replaced every occurance of "date('r')" with
"strftime('%a, %e %b %Y %T %z')" in ..../horde/imp/compose.php.
And yes it worked straight away..................