Solr9 indexing crashes retain-tomcat

One of my customers has a Solr 8 index of 3.500.000 items. With about 0.01 % failed (286 items). I upgraded the Retain software to version 23.4. Then started the Solr9 indexing. The indexing crashes retain-tomcat service several times during the indexing. When I restart the service from the commandline the indexing continues again. Now I am almost ready and the system recommends to switch to Solr 9.

Does anyone have the same experience with Solr9?

I have checked logfiles for a reason of the crash, but did not find anything. Checked catalina log, EmbeddedSolrLatest log, Indexer log for errors.