Hi Loggy,
We use the WordPress core PHP api to add htaccess rules to the .htaccess file. We do not use fopen or fwrite though. I am under the impression that all WordPress installs regardless of the server makes use of .htaccess rule to operate permalinks. I'm not positive though. Maybe catch me up on how this works?
In the next version we've added a flush permalink rules to the deactivation hook so this should take care of lp's contribution to rewrite rules and clean the htaccess file.
@Carlo,
Try deleting and updating the latest version. I don't see our rules in your htaccess file and the file does not look corrupted so I'm not sure off the bat why the 500 error is throwing.