Comment # 3 on bug 1190524 from Rainer Sabelka

Recently I found

https://www.reddit.com/r/kde/comments/mebx8f/reminder_to_check_on_your_baloo_from_time_to_time/

and did like suggested in this article
 balooctl disable
 balooctl purge
 balooctl enable

The index finished rebuilding over night.

> balooctl status
Baloo File Indexer is running
Indexer state: Idle
Total files indexed: 668,289
Files waiting for content indexing: 0
Files failed to index: 0
Current size of index is 7.88 GiB

The index size went down to 1/10 but curiously also the number of files
indexed. (The number of files indexed is plausible, a "find $HOME -type f"
return ca. 1.000.000 lines).

So for me, the problem seems to be fixed.
I guess it will be difficult, however, to find the cause of the problem in
retrospect.