To fix it:

\b2evolution\blogs\inc\_core\_param.funcs.php
Line 1367:

	else
	{
		if( ! empty($base_tag_set) )
		{
			if( isset($Debuglog) ) $Debuglog->add( 'regenerate_url(): Using full URL because of $base_tag_set.', 'params' );
			//Replace next line, i commented out. It still works for me, not testet on other hosts.
			//$url = $ReqHost.$ReqPath;
			$url = $ReqPath;
		}
		else
		{	// Use just absolute path, because there's no <base> tag used
			$url = $ReqPath;
		}
	}

Thanks Tarion! It's really cool when people find a solution to their own problem then share it with everyone.