Something stupid happened: I wrote an email and wanted to cut some text out with <strg><x> but the mouse was in the KMail-window and so a mail-folder was cut out. I did undo but the folder is still gray and can't be activated. There are 400+ emails in it. Maybe I can restore them from backup but how to re-activate the folder?

I checked the source and it looks like the messages still exist in ~/.local/share/.local-mail.directory/.Posteingang.directory/web.de/new/. I also tried to close and open Kontact. Any suggestions? Please!

After rebooting this morning, all works fine again. I don't know why a restart didn't change anything but a reboot did but whatever, it works now.