Galera MariaDB not joining with IST
My own hint with the Write Set Cache Size (aka Galera Cache Size) nailed it (I should have asked earlier, it seems some supernatural voice told me the answer.)

However I do not quite understand why 15 GBytes were too few for 45 KBytes/sec for an 18 hours transfer after a next try, obviously it was only a bit too short... (In my question I wrote about 10 hours, which I found out was not true).

Changing the transfer method to rsync finally fixed it as the transfer took only 1 hour.