Help needed: Feedburner problems after relaunch

(see German version of this post here)

As some feed subscribers may have noticed, our three already relaunched blogs running WordPress 2.6.1 (medienlese.com, netzwertig.com, imgriff.com) are showing occasional feed problems: Sometimes, instead of the feed, a completely empty file is delivered by Feedburner.

We have made several attempts to locate and solve the problem, so far without success. Here’s what we noticed and tried so far:

• Feedvalidator.org does not find any errors in our Feeds.
• We think we can rule out charset/encoding problems. Our feeds are UTF8-coded and announced as such by the server .
• When the problem occurs, not all Feedburner servers deliver empty feeds. (In the HTTP response headers, the current Feedburner server is listed.) Then again, there’s no consistency in which servers are affected.
• FeedBulletin, the Feedburner monitor feed, lists no errors; also the Feedburner profile shows no indications for our feeds to be erroneous.
• A manual resync triggered in the Feedburner profile will help, but not for long. A manual Feedburner ping will help only sometimes.
• The redesign involved upgrading WordPress from 2.5 to 2.6.1; other than that, there should be no differences regarding the feed. We checked all installed plugins. None of them (whether downloaded or written by us) modifies the feed (except, of course, the Feedburner “FeedSmith” plugin which we left untouched).
• For a while, we have deactivated the eTag and LastModified-HTTP-Header in the WP core, thus keeping the clients from caching. Since that didn’t show any improvement, we rolled back to the original WP code.

Trying to contact Feedburner directly and via the Google Group has not been successful so far.

Has anybody encountered this problem, or does anybody know how to get support from Feedburner?

In any event, we apologize to all subscribers who have been getting the empty feed. We hope we will find a solution soon.