Excellent, works now as expected.
Now its getting more difficult because when loading a large amount of records each index segment gets a ".del" file of different size.
Nevertheless all data is loaded without loss and an optimize will clean up everything.
Obviously another bad side effect of multi-threading and NOT seen with version 3.5.
Will be hard to find out what happens.
An overlapping of solrwriter?
Does it make sence to spend more time looking into this if multi-threading will be removed anyway in 4.x?