'Unknown error' caused by redirect in htaccess I was getting an 'unknown error' when trying to access the admin. I couldn't see anything in the Craft logs apart from 404 related errors.
I replaced the app folder but still no luck, eventually I discovered it was a rewrite rule in my .htaccess causing the problem.

# redirect PHP or HTM to no extension
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^\ ]+)\.(php|htm?)
RewriteRule ^ /%1 [L,R=301]


I'd love to understand why, and how I can get around this in the future?
It might be down to the order you have the code in your htaccess file? Did you try putting your code above the Craft rewrite code?

# redirect PHP or HTM to no extension
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^\ ]+)\.(php|htm?)
RewriteRule ^ /%1 [L,R=301]

# Send would-be 404 requests to Craft
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^/(favicon\.ico|apple-touch-icon.*\.png)$ [NC]
RewriteRule (.+) index.php?p=$1 [QSA,L]


Also I tend to exclude certain directories when I don't want rewrites to apply. One way to do this would be;

# ------------------------------------------------------------------------------
# | Exclude directories from the rewrite rules                                 |
# ------------------------------------------------------------------------------
RewriteRule ^craft - [L]