HTTP ERROR 500 and 404 NOT FOUND

[newuserhere]

Hello,
I just installed Ultimate phpBB SEO Friendly URL 2.0.0-b6 on a phpBB 3.3.0 board and I cannot make it work.
I did the few steps:

• Activated URL rewriting with Advanced URL rewriting type;
• Configured URLs in Forum URL Management section (The cache directory was successfully found / The cache directory is writable);
• Copy the .htaccess from Server Config in .htaccess fine on my server.

But the problem is that (i) I get 404 not found on forums/topics/posts and (ii) everytime I enable/disable the Extension, my board get HTTP ERROR 500 for like 20-30 mins.

Later edit:
Now I'm getting:

Server Error

403 - Forbidden: Access is denied.
You do not have permission to view this directory or page using the credentials that you supplied.

Later edit 2:
I run phpBB on Windows server btw.

[Meis@M]

Hello and welcome
u should use "Config Scope:" option and "More Options:" then "RegEx Left Slash:" in "Server Config" section to copy right .htaccess codes.

after copy pure the cache from acp main menu.

[newuserhere]

Thank you for your reply!
I did exacly as you said but I'm still getting:

404 - File or directory not found.
The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.

What am I doing wrong?

[Meis@M]

if possible send me ur access root and ACP for testing by me

[newuserhere]

Thank you! I will send you a pm with the details.
Have a nice day!