Fatal Kmail error after upgrade

I'm upgrading an old system running KDE 4.14, to 20.08. The mail is stored in ~/Mail (which is where I want to it to remain). After a lot of messing about, I finally discovered the Akonadi Maildir resource which showed the mail folder structure under ~/Mail. I then deleted the default Local Folders resource, since it had no obvious use alongside the Maildir resource.

Since then, Kmail displays a fatal error soon after starting, every time:

"The email program encountered a fatal error and will terminate now. The error was:
Timeout trying to get lock."

Any ideas on how to stop the error happening? It makes Kmail completely unusable.

I've tried reinstalling the Local Folders resource, but it makes no difference.
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David Jarvie
KAlarm author, KDE developer