My German isn't good enough to understand those articles, and I'm finding it a bit tricky to understand the Google translation. The jist I'm getting from those two pages is that the access provider isn't liable for what is transmitted if the transfer is initiated by another user.

I think the bit I'm referring to would be covered by § 15 paragraph (4), which Google translates to "To comply with existing legal, statutory or contractual retention periods, the service provider may block the data." I think "block" here means "store", in the sense that the user data which includes "Information on the beginning and end and the extent of each use" (paragraph 1) needs to be recorded (and retained) for a period of time required by a different statute.

I feel like I only understand about 30% of those documents, I might make more progress with a better translation!

http://www.gesetze-im-internet.de/tmg/__15.html