Ok, at last I got hit by the same problem: Each time I start kamail, one index file wants to be rebuilt. Please not, not all index files, just one. Deleting all *.index and *.index.ids does not change anything.
What I did: 2 days ago: dist-upgrade --> everything works as expected, not strange messages. Just some hours ago: dist-upgrade pulled in some TDE updates --> index files of one folder gets rebuilt when I start kmail.
Nik