Hi all,
I have started using Kontact again after a long time and at the moment I am loving it. I even found out how to make a backup of everything there is in Kontact.
I use the program pim setting exporter for that. It's part of Kontact.
In there I choose to backup everything, default setting, and a few minutes later I have a 3.5GB file with everything.

As I wrote above I used the program PimSettingsExporter but I have stopped doing that. I exported all there is to export using that program and got one huge zip file, thinking all is well.
Well, after installing a new OS I really thought I could just use the program again to import all Kmail data. Well, no!!!
It imports almost nothing, certainly no mails. So I was afraid I had lost it all.

But, it Kmail also has it's own backup system, hidden in Menu -> Settings -> Configuring Automatic Archiving.
Here you can setup backup schemes for each folder in Kmail. It can include all subfolders, so you don't need too many backup schemes.
I also did that on the previous OS, so I had 2 sorts of backups.

In the new OS, I recreated my e-mail identities and accounts with the network connection closed down to prevent new mails coming in before I had finished restoring the data.
Restoring is not what you would expect. You can not use Menu -> Tools -> Import/Export Kmail data cause it won't restore your data, just as PimSettingsExporter did not do that.
You need to use Menu -> Tools -> Import from another client. In the first windows you need to close Kontact, in the second (which was empty here with me) I chose in the lower left corner: Manual selection.
In the next window, in the blue bar in the center, I used the default value: Import KMail Archive file. At the bottom you need to select a folder in Kmail where you want your data to be stored. After that click Next and choose the backup file. Data will then be restored.

Mind you, this is only e-mail data. No filters, no calendar, no addresbook, just mails.
For everything else you need to backup things separately. I just wished, the complete data export in Kontact would better, but I couldn't restore my data with it and had to do a lot of things manually.