Some Kmail accounts fail

After a software update around 6 June, I have problems with 2 of 4 IMAP 
accounts.
Those accounts are with the same email server.

The error message on trying to update an inbox folder is “Unable to 
fetch item from backend (collection -1):Unable to contact resource”
The accounts are coloured red.
I’m unable to access the settings for those accounts. The receiving 
entry shows "Retrieving folders (0%)’
Attempting to modify the receiving account settings results in a very 
brief display of a small box - too fast for me to read the contents.

I can use the wizard to establish the accounts again, and on doing so 
the account seems to be established normally, mailboxes and contents are 
established, but shortly after the  same behavior returns

I’m using an up to date version of Neon, Kmail version 6.1.0 (24.05.0).
dpkg.log tells me: 2024-06-06 10:31:06 status installed kmail:amd64 
4:24.05.0-0xneon+22.04+jammy+release+build63

The problem is not experienced when using Thunderbird to access the same 
accounts and I can reproduce the fault on another machine.

Does a schema of the conceptual architecture  exist? - it appears 
Thunderbird and Kmail share common elements
Do kmail logs exist and if so where might I find them.

Thanks
Tony Bazeley