Followup.
Found this message in server log:
/var/www/sites/[…]/wp-content/plugins/download-manager/.htaccess:
Invalid command ‘IndexIgnore’, perhaps misspel
led or defined by a module not included in the server configuration
Autoindex module is disabled on Apache server, so IndexIgnore crashed the whole thing.
I commented the IndexIgnore command in .htaccess and everything seems to work fair.
EF