Dear Monks,
I've found the reason of the differences.
In Debian versions Apache in configured with

# Default charset to iso-8859-1 (ttp://www.apache.org/info/css-securit
+y/).
                                                                      
+          
AddDefaultCharset on
[download]

which completely ignores the encoding given in META tag and uses always the default encoding. In Apache installation under OpenBSD the parameter was not present and so was correct.
When I removed that nasty parameter everything worked on Debian also...
Hope it helps you, too!