For us and for many other people in the world the RSS-feed from BOINC does not work any more.
There are compliants in the BOINC message board but none of the staff members seems to show any interest neither to take action.
What happened?

Works fine for me

If you are referring to this URL:

http://boinc.berkeley.edu/rss_main.php

then yes, it is currently inoperable due to a simple German character code error (RSS markup has difficulty with foreign characters when they are rendered as code). My guess is that the Berkeley team just hasn't noticed it. This is a very easy error to fix; obviously, just replace the character code (&uuml with something else, maybe just a simple "u".

or just do things properly, and serve it as UTF-8, and wrap the contents of <description> in CDATA tags, that would solve so many problems, and allow them to use HTML in <description>s

or even better would be changing to the Atom format, RSS is a bit out dated

as a web dev and someone who prefers to get my news via RSS i'm working to get the feeds up to scratch, first thing is to get the basics sorted (especially the nasty <guid> problem)

What exactly should <guid> contain?
Right now in my feeds I put there an URL to specific content. This should be unique identifier from my point of view, but is it right?

I think the guid just needs to be a unique string to identify an item, and a URL to the item works.

if you read the spec for the format of the feed you're using it'll probably explain better, but in short Marty-UK is right
for RSS at least (i'd have to check for Atom thou) <guid> is a sub-element of an <item>, preferably one <guid> for each <item>

the idea is to be a globally unique identifier, and thus the most important thing is that they never change
if you have "archives" which use permanent URLs then you can use the permanent URL as the GUID by specifying <guid isPermaLink="true">%URL to archive page here%</guid>

if you don't have archives, or for any other reason can't use a permanent URL, you can use and string as the GUID, but again, it must not change
to use a non-url guid, use... <guid isPermaLink="false">%string goes here%</guid>

considering they've got to be globally unique, but never changing, it might be an idea to use part of your domain name, or your own name prehaps

for example i use <guid isPermaLink="false">carre_##</guid> where ## is a number, which is handy to know how many items/entries i've posted, but it can be anything, and doesn't even need to use the same format for each item, they can be different lengths, and/or totally random strings, as long as they don't change, and are correctly labeled as permanent URLs or not

run your feed thru the FEED validator to check that it's valid too (there's also a verson hosted by the W3C: W3C Feed validator, it's the same code, (available from sourceforge.net) just incase the feedfalidator.org site is down (which happens sometimes)

something else that's handy is <pubDate>ISO date/time</pubDate> elements (the feed validtor will tell you if the format you use is correct, ISO dates have specific formats), so that feed readers know when an item was published (rather than a bunch of them all having the same date/time)

but like i said, read the specs, as they'll tell you exactly how things should be done...
RSS 2.0 Spec - Atom Spec

for anyone who's interested in hosting a feed, for a blog, or news (or anything else) i strongly recommend the Atom format, it's much better than RSS, which is quite outdated these days, Atom is modern and solves many of the problems with RSS