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 :-)