I got this SMF forum for my wife and is working very well. I am very satisfied with SMF.
However, sometimes you get into trouble with it and the worst part is you do not have much logs to work with. Also, apache logs won't help much either.

So, below you will find things I encountered.

1. 500 error without any useful errors in the logs (apache or SMF):

I busted my head a bit with it but it turned out it is quite easy: delete all cache files from cache directory but please make sure you do not delete .htaccess file. That one must stay there.

I know is not much here but I was adding it hoping someone will really need it.
Cheers!