@gt I did some experimentation how to get my old messages and contacts back. Here is the solution, which works (with a little draw-back):I used the new re-registered Axolotl profile (all files in .config/textsecure.nanuc and .local/share/textsecure.nanuc) as basis. Then I replaced the database db.sql file with the old db.sql from my December back-up. (Apparently, Axolotl deletes all old messages from the db.sql after re-registration. On the other hand, attachments remain in the profile). Then I copied the contacts.yml from the back-up to the config folder of the new profile.
This works fine, I see all my messages and all contacts. The only draw-back is that I can only write a message to a contact AFTER this contact has written a new message to me. Apparently, the other person has to re-initiate the secure connection. I can live with this draw-back, as it is a matter of time that all (relevant) contacts will be working again.