Woocommerce internal server error on checkout

I am experiencing this weird problem with my htaccess file everytime I tried to checkout on my woocommerce site. This is the htaccess file I currently have..

# BEGIN REMOVE SLUG CUSTOM POST TYPE RULES
RewriteRule ^slider/(.+)/$ /$1 [R=301,L]
# END REMOVE SLUG CUSTOM POST TYPE RULES


# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

but its strangely become like this after I checkout

# BEGIN REMOVE SLUG CUSTOM POST TYPE RULES
RewriteRule ^slider/(.+)/$ /$1 [R=301,L]
# END REMOVE SLUG CUSTOM POST TYPE RULES


# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
Rewrit

and 500 server error prompt up instead of showing thank you page but its all working if I fixed the htaccess file manually and refresh the same page, thank you page will show and also order was recorded fine even if I don’t fixed the htaccess file.

I created my own woocommerce child theme for this site and I am using only two plugins called “Remove slug from custom post type” and “SEO Ultimate” to make the product and category urls as I needed and both plugins seems to be changing htaccess on the fly, but I am not really sure why only on checkout it makes htaccess file cut off like that…? These are the url where I’am getting this error

http://mysite.dev/checkout/order-received/709?key=wc_order_53db13e068104

its redirect to this url after I correct htaccess manually.
http://mysite.dev/checkout/order-received/709/?key=wc_order_53db13e068104

also this error doesn’t occur all the time please help!