Disclaimer: this is most likely not a bug in kate, but I don't know what package to assign this bug report to. Please reassign appropriately. Every time I open the 'file open' dialog in kate or a number of other KDE apps such as kile or okular, I get a little error widget saying 'Cannot open folder tags:/' (My own translation: My German GUI actually says 'Ordner tags:/ lässt sich nicht öffnen.' ) If I click on the OK button the 'file open' dialog appears, and there are no further problems.
I forgot to mention that the following message are printed at the command line when the message widget opens: [17::40:54.472] unknown: PostingDB::open MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: PositionDB::open MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: DocumentDB::open docterms MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: DocumentDB::open docfilenameterms MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: DocumentDB::open docxatrrterms MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: IdTreeDB::open MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: IdFilenameDB::open MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: DocumentTimeDB::open MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: DocumentDataDB::open MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: DocumentIdDB::open indexingleveldb MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: DocumentIdDB::open failediddb MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: MTimeDB::open MDB_NOTFOUND: No matching key/data pair found [17::40:54.472] unknown: dbis is invalid [17::40:54.472] unknown: tag fetch failed: "Failed to open the database" [17::40:54.472] unknown: "tags:/" list() invalid url [17::40:54.472] unknown: "Ordner tags:/ lässt sich nicht öffnen."
I found an upstream bugreport: https://bugs.kde.org/show_bug.cgi?id=437176 The workaround mentioned there (balooctl purge) worked for me. Thanks for your work on Debian.