An error 500 is caused by an internal PHP/Apache issue. Do you have any additional error output in apache error log, showing a fatal error or a segmentation fault?
Please, ask to your hosting support if they can gather more information about the error. Just in case, you can disable .htaccess and test a case to check if error 500 is related with its configuration.
thank you for the info.
sadly, that workaround does not work in my case.
i updated php to 5.4.37 on my development server and still i get the message about "500 error" using the composer, while apache log does not trace any 500 error, still, i get only this notice:
"PHP Notice: Undefined property: F0FTable::$autotweet_composer_id"
[and so on, as prevoiusly posted]
i bet it's something related to adminTools. but i have no time to play with that, right now (actually, i don't need the composer. mine was just sort of debug info for your :-> ).
if i'll manage to fix that in the future i'll post my solution.
Yesterday, we released AutoTweetNG v7.7 stable. Please, upgrade to this version to check it.
Additionally, we run AdminTools PRO in all of our servers and we haven't found a similar issue with PHP 5.4 or 5.5. Just in case, you can simply disable .htaccess and AdminTools system plugin to check if it is causing error 500.