@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.

I think that phone numbers are the only sensitive data yes. Not sure the logs are still useful.

@Fla In Teleports, I don't see a search box where I can search for groups. I can only enter a contact by phone number. Or is the option hidden somewhere? The main screen is blank with a menu in the left top corner, with saved messages, contacts, settings, night mode and about. I could't find a manual for Teleports. I've gone through the menu items, but don't see how?

Okay, I will contact @nanu-c about website.
I will mark this question as solved. Thanks!