This fixed itself, after a while, but began again. Then stayed fixed. So far.
Initially emails could not be deleted, and still showed as unread after being read. Then the mail arrival indicator showed that emails were coming in, but they did not appear in Kmail. So I rebooted. Now no folders are shown, and the filter configuration can't see them either.
All email sources are affected, ISP, Gmail, and blog.
Update. I sent, using kmail, an email to my gmail account. This triggered the gmail folders to reappear, and the blog email folders, but only inbox, outbox (I don't recall that one before), and sent-mail of the local folders - with complete contents. I don't leave anything in the ISP folders, which have not reappeared yet.
Now another local folder has popped into existence. But there are many more still not showing.
Sent an email as blog address to ISP address. The ISP folders have now popped up. But there are still several emails on the server, claiming to be unread, but they have been downloaded to kmail and should have gone from the server.
Trashing an email caused the trash folder to appear.
If an existing filter sends an email to a folder, that folder appears.
I expect that if I copy an email from one Akonadi folder to another, that folder, now having a new email in it, will appear. But there has to be a better way.
Update - On booting up next morning all folders were again visible.
But later in the day the cycle begins again.
Closing Kontact and re-opening as soon as the emails stopped arriving got them running again, but the filters don't work. I'll see what happens in the morning.
Update - All working. No emails stacked on the server.
Initially emails could not be deleted, and still showed as unread after being read. Then the mail arrival indicator showed that emails were coming in, but they did not appear in Kmail. So I rebooted. Now no folders are shown, and the filter configuration can't see them either.
All email sources are affected, ISP, Gmail, and blog.
Update. I sent, using kmail, an email to my gmail account. This triggered the gmail folders to reappear, and the blog email folders, but only inbox, outbox (I don't recall that one before), and sent-mail of the local folders - with complete contents. I don't leave anything in the ISP folders, which have not reappeared yet.
Now another local folder has popped into existence. But there are many more still not showing.
Sent an email as blog address to ISP address. The ISP folders have now popped up. But there are still several emails on the server, claiming to be unread, but they have been downloaded to kmail and should have gone from the server.
Trashing an email caused the trash folder to appear.
If an existing filter sends an email to a folder, that folder appears.
I expect that if I copy an email from one Akonadi folder to another, that folder, now having a new email in it, will appear. But there has to be a better way.
Update - On booting up next morning all folders were again visible.
But later in the day the cycle begins again. Closing Kontact and re-opening as soon as the emails stopped arriving got them running again, but the filters don't work. I'll see what happens in the morning.
Update - All working. No emails stacked on the server.