All forum messages between 15.8. and 4.9. were lost. I was unable to recover any data, as it seems that JForum had stopped writing data to the database around 15.8. and kept everything only in memory. Yes, it’s a severe problem.
I made some changes to the JForum database parameters last week so that it should be more eager to write its data to the database, but did not quite yet make any software updates because of migration problems with the database. The forum will hopefully work ok, as it has worked most of the time before, but there is a possibility that some data loss occurs again. We’ll make more proper upgrades soon.