Hello, thank you for your prompt reply.
Actually, I thought all configuration files were written correctly, but who knows

The point is that I got the error message after entering the user and password, i.e. the index.php file in my admin folder loads correctly initially.
If that may help, please find attached the config.php files as well as .htaccess file (located in
http://liturgix.wwwmi3-ss14.a2hosted.com/lit154):
=========================================================
# Prevent Directoy listing
Options -Indexes
# Prevent Direct Access to files
<FilesMatch "\.(tpl|ini|log)">
Order deny,allow
Deny from all
</FilesMatch>
# SEO URL Settings
RewriteEngine On
# If your opencart installation does not run on the main web folder make sure you folder it does run in ie. / becomes /shop/
# RewriteBase /
# RewriteRule ^sitemap.xml$ index.php?route=feed/google_sitemap [L]
# RewriteRule ^googlebase.xml$ index.php?route=feed/google_base [L]
# RewriteCond %{REQUEST_FILENAME} !-f
# RewriteCond %{REQUEST_FILENAME} !-d
# RewriteCond %{REQUEST_URI} !.*\.(ico|gif|jpg|jpeg|png|js|css)
# RewriteRule ^([^?]*) index.php?_route_=$1 [L,QSA]
RewriteBase /lit154/
RewriteRule sitemap.xml /index.php?route=feed/google_sitemap
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^?]*) index.php?_route_=$1 [L,QSA]
============================================
Any ideas what to do to resolve the problem.
Your help and time are appreciated!
Milka