I have identified what is happening for me and a means of managing it, but not the cause or ongoing solution - others with more expertise than this old man needed for that.

If I shut the computer down with Thunderbird still active, when it is restarted the connection to the server is established correctly. However, if I close Thunderbird before I shut the computer down and it is restarted, the local directory link to the server is not re-established - leaving Thunderbird open and then restarting the computer again resets the link to the server correctly.

Somehow the closing of Thunderbird prior to shutdown deletes the connection even though it is still existing in prefs.js

I suggest the change is in the use of the snap programs used in the newest version of Ubuntu and I would think also by your close clone of it.

See https://ubuntu.com/core/docs/security-and-sandboxing

Thanks for your response Matt. I had a look at the link but its a bit too complex for me. I guess I'll just have to hope the next update of the snap will address the problem and live with it in the meantime.

Snap updates have not resolved the problem which persists but I have now established exactly what is happening..

If I boot the computer at the beginning of the day Thunderbird starts but the network connection to my server is not made and the emails are not displayed. If I then reboot the computer Thunderbird again starts but the connection is made to the server correctly and all emails are available. Shutting down the computer at any time during the day and restarting Thunderbird always connects correctly.

However, the next day the same problem returns - no connection on the first boot but on reboot everything connects correctly - even if the is an 8 or 9 hour delay between reboots.

So on each new day the problem returns. The problem seems to be somehow time/date related.

Any thoughts?