Hi,
Did you tried the following:
1) Check if your .htaccess file is writable. This can be different from server to server, but usually require rw-rw-r– / 0664 Also ensure the file owner is the same group as php
2) Sometimes the other codes wrongly use the flush_rules() which hijack the default filters for rewrite. Try to disable the other plugins to figure out which one produce the issue.
3) De-activate and RE-activate the plugin, apparently worked for some users.
4) Create a backup of .htaccess, then delete it from server. Go to Settings > Permalinks > update once, this should create the file again on the WordPress root. Try to change any WP Hide options which should change the .htaccess content accordingly.
Thanks