I think it is related to this:
Warning: preg_match(): Compilation failed: unmatched parentheses at offset 11 in /web/htdocs1/stalemladicz/home/www/wp-includes/class-wp.php on line 231
Please check your permalink settings. Do you have any plugins enabled that influence permalinks?
You can use this lost password url: http://www.stalemladi.cz/wp-login.php?action=lostpassword
But somehow the lost password url got replaced by “6)oqWxLQAo10Jl7PVQbQzpX” which causes a regex failure in class-wp.php. How did you change the lost password url? Script in functions.php? Plugin?