Can you at least provide us with a list of hacks and plugins tou tried? I am afraid you gave not enough clues to the cause of your problem.

Good luck

I really just need to know what files are involved in generating the RSS feed. The code they show me looks OK (see http://freesite.iblogger.org/index.php?tempskin=_rss) Nevertheless I have installed these since I last checked RSS and round it working:

TinyMCE (I think this is what the WYSIWYG is called :))
Lightbox
Avatars
Tag Cloud
Social Bookmarking
AcroLink

I'm guessing we can rule out AcroLink, Tag Cloud, and Lightbox as the problem. Probably edited something slightly wrong when setting up avatars or social bookmarking. WYSIWYG is disabled so it wouldn't be that.

You're right, but I needed a starting point.
For the XML look at the xmlrsv folder.

Somehow your first line is blank. That may cause the trouble. But I have no clue what causes the first line to be blank.
The second line is the xml declaration and that looks ok to me.

<?xml version="1.0" encoding="iso-8859-1"?><!-- generator="b2evolution/1.10.2" -->

I was looking through some of the files I modified, and I found this instruction:

In skins main.php

* - Just delete the Social Bookmark Services (SBS) you don't need.

# 6.1.1: Find
<?php link_pages() ?>

# 6.1.2: Add below

<?php if( $disp == 'single' ) { ....etc...

The problem is that the line there was actually:

<?php link_pages( '#', '#', 'number', '#', '#', '%d' ) ?>

I'm not sure what the # and number additions are, but it appears that they are probably necessary for something else. I can't remember if I edited this line for one of my other installations.

This doesn't sound like the source of your problem. I run through the things you filled in ad link_pages:

void link_pages( [ $before = '#'], [ $after = '#'], [ $next_or_number = 'number'], [ $nextpagelink = '#'], [ $previouspagelink = '#'], [ $pagelink = '%d'], [ $more_file = '']  )

From the [url=http://doc.b2evolution.net/]Technical Docs[/url] it just sais you passed default variables. That means link_pages() does exactly the same as your code.
This only to ease your mind and point you to the guru stuff.

I wonder if there is any place where you can find a line break of any kind that can explain the first empty line in your rss. My first line is:

<?xml version="1.0" encoding="iso-8859-1"?><!-- generator="b2evolution/2.0-alpha" -->

and that's the first line and not the second.

Do you have this behavious at rss only or also atom and rss2?

The # is empty or default default (pass what is supposed to pass). Number is a string and %d is the actual number, which will change according to the pagenumber.

Guru alert: explanation is from non guru.
number is probably a choice between an actual number or a preset text.

I checked the post and comment feeds for all RSS versions and atom. I see a blank first line in all of them.

./xmlsrv/xmlrpc.php on line 60:

// Trim requests (used by XML-RPC library); fix for mozBlog and other cases where '<?xml' isn't on the very first line
$HTTP_RAW_POST_DATA = trim( $HTTP_RAW_POST_DATA );

so I guess I was right on this first line being the problem.

It seems like that's already there and intact. What other steps would this RSS data pass through before it gets to the feed URL?

Your rss/atom feeds are generated by _main.php in /skins/_rss/ , /skins/_rss2/ , /skins/_rdf/ & /skins/_atom/

You want to check all those files for any whitespace before the xml declaration

¥

I'll look.
I haven't modified any though, and since all the feeds are screwed up it would come from one file. I see a bunch of whitespace before the first tag on my main page also, could these be related?

EDIT-
I checked, they all look fine to me. First output is:

echo '<?xml version="1.0" encoding="'.$io_charset.'"?'.'>';

No whitespace

This guy had a similar problem:
http://forums.b2evolution.net/viewtopic.php?p=37082&sid=b6ce4ac42af1773ab131fa847bb04bba
I'm checking out config.php!

No luck...

I have noticed that I have several lines of whitespace before the first line of my regular blog pages (IE index.php). Could this be related?

Found a line of whitespace under conf/_advanced.php
And at the end of conf/_application.php
Looking more

I deleted these lines, didn't seem to have any effect.

One last idea - Is there a piece of code I can add to all the feed skin files that will delete the previous line or something?

From the looks of things you have whitespace in one of your files in /conf/ or /inc/_main.inc.php

¥

I'll checked it out again. Found one blank line at the end of _stats.php. I deleted it and no effect

I'll check the files I modded for the social bookmarking again. Maybe I missed soemthing, it's the most likely thing

SUCCESS!
Finally found it, a blank line at the end of /inc/MODEL/items/_itemlist.class.php after the ?>. Can't believe that took me half a week to solve!

Well done.
Yes, I do believe it can take half a week looking for literally nothing (blank) ;)