I looked for help and FAQ pages on your Host and did not find any information about .htaccess files. I did a Chat Session with one.com Hosting support and one.com Hosting does not allow the Options directive to be used in .htaccess files so you will need to comment it out by adding a # sign in front of that Directive in your Root .htaccess file.
# DO NOT SHOW DIRECTORY LISTING
# If you are getting 500 Errors when activating BPS then comment out Options -Indexes
# by adding a # sign in front of it. If there is a typo anywhere in this file you will also see 500 errors.
Chat session with one.com Support (great support):
You are now chatting with 'Niksaan'
Niksaan: Thank you for using One.com 24/7 Interactive Online Support. My name is Niksaan. How may I assist you?
you: Hello do you allow .htaccess files and do you allow the htaccess file permission to be set to 404 instead of 644 file permissions?
Niksaan: yes, we do allow .htaccess file, but it depends on the script you will place in the .htaccess file
Niksaan: and you can set its permissions to 404
you: I am assisting one of your customers here - http://wordpress.org/support/topic/internal-server-error-after-secure-ht-access?replies=3
you: Ok then do you allow the "Options" directive in htaccess files? example Options -Indexes?
Niksaan: no, we do not allow Options
Niksaan: you need to comment out all options line
you: ok yep i thought that might be the issue. great thank you
Niksaan: by placing a # infront of these lines
Niksaan: You are welcome. Feel free to contact us again for any further assistance. Bye and hope you have a nice time : )
you: yep i will inform your customer to do this. # DO NOT SHOW DIRECTORY LISTING # If you are getting 500 Errors when activating BPS then comment out Options -Indexes # by adding a # sign in front of it. If there is a typo anywhere in this file you will also see 500 errors. Options -Indexes
you: have a nice day!!!!!!
Niksaan: yes, this is correct :)