ok finally I got my conversation history back. I don't know exactly what was happing. Maybe it is because I got a sms-message just after the update but before I tried to restore the conversations. In the .rtcom-eventlogger directory were my backup.tgz, el.db (with old history) and el-v1.db (with one new message). To get my old conversation history back I had to remove (or move away) the el-v1.db (but don't start the conversation app), restore from backup (this creates a new el-v1.db). And now all conversations are back :-)