Czesc Jerzy,

Without access hard to guess, have you checked Access settings for menu item and that page that should be used there? And yes, you forgot mention about full template name.

I enclosed the details in the hidden content

Thanks, look down, you forgot about that

Sorry, my fault, below

I saw it's OK now.

Thank You. Do we know what was that? Just in case of re-installing everything I don't want to bother You.

Unfortunatelly the error still occurs when I open the main page...

I am not sure why - I anclosed another account

I updated serveral extensions on your site + Joomla itself, in general it was your duty.

And I couldn't reproduce that issue anymore.

BTW

Remember to install SSL for that domain.