Tags and Categories and Search not working

yesterday working on my WP homepage layout I had a crash: tags not working, category search not working.
Searching the forum I found many desperate notes around this problem, mostly saying “new installation of everything solved the problem”.

This is why I want to share my solution:

I found out that also search was not working any longer, not just Tag and category.

For “SEARCH not working” another common problem in WP Forums I found the solution earlier.

some programs – mostly the caching programs – mess around with .htaccess. They create their own version of .htaccess and add the standard WP-Part leaving out the line “DirectoryIndex”. If this line is missing my search stops working.
This may be a problem just for some webspace providers, otherwise the plugins wouldnt do that.

Now funny enough, to add the line “DirectoryIndex” into .htaccess also solved my TAG/CATEGORY Problem. I do not understand why but wanted to share to save others some time and hazzle.

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