Here is a screenshot of the error message...

Hi That may mean that you have multiple accounts and devices all connecting to the same email service at once. The email provider's server often set a limit on how many connections you can have a to one time. Mobile devices are particularly likely to create multiple connections. In Thunderbird account settings you can set a low number to restrict how many connections Thunderbird can make, but you also need to log off email and shut down all other devices that connect to your email, for 10 mins or so to let the server reset. This earlier question has a screen shot of the setting to change in Advanced Account Settings. https://support.mozilla.org/en-US/questions/1327317

Thank you Agnes, but I am not an "IT professional" and you might as well be writing in Japanese for all I could make of what you said. Would it be possible for you to explain to me in simple terms what I need to do to stop getting that strange error message?

I made it about as simple as it can be. Go to Account Settings - advanced IMAP settings (assuming you are using IMAP) - and find the cache setting that I said was in the earlier question - you looked at that link? Here is an article with pictures - look at Advanced IMAP settings - server settings to cache https://support.mozilla.org/en-US/kb/configuration-options-accounts Set it to 2 or 1. Save. Shut down Thunderbird. Log OFF from email in every computer or mobile device you use for email. Then shut every computer and mobile device off - right off - even if you are not using them currently for email. Power off, not sleep. Leave them off ten minutes at least. Then start the Thunderbird computer and see if you can operate normally.

Honestly, you lost me.

Well, try one step at a time and see how for you get and then respond with the specific part you are stuck on.

I don't even understand the language you're using

That is why I sent a link to the page with pictures. Read the page carefully. Each step is illustrated. Pay attention to the section with the heading "Advanced IMAP settings". Follow the directions on that page to open Thunderbird account settings as per the pictures. If you can't follow that you will need someone to help at your computer. Surely you understand about turning off every computer or phone or tablet that might possibly connect to email. Do that part anyway.

Surely YOU know what "Winformation overload" means, and understand that elderly people are often totally bemused and overwhelmed by computers and their language?

I do not know what it is you do not understand. You might get part of it but not other bits. You appear to be lost right from the first word, so I'm assuming you never set up your own mail account and have no idea what or where to locate 'Account Settings'

Instruction: Go to 'Account Settings'

How to get to 'Account Settings':

Use one of the following methods - both are valid - use whatever is easier for you. Method 1: Right click on the imap mail account name located in 'Folder Pane' and select 'Settings' OR Method 2 click on the imap mail account name located in 'Folder Pane' to see helpful information on the right side where you click on 'Account Settings' located top right.

Then 'Account Settings' will open in a new tab. See a couple of images below which explain what to click on.

Instruction: go to Advanced IMAP settings - server settings to cache

How to find that location. My previous comment asked you to 'Go to Account Settings' which would open in a new tab. You need to select the 'Account Settings' tab to see what it contains. On the left side will be the name of the mail account. Left click on 'Server Settings' which is just below the name of the account. On the right side you will see a 'Advanced...' button - left click on that button

It will open another smaller window called 'Advanced Account Settings' Look for this line: 'Maximum number of server connections to cache' By default, this will be set to '5' left click on the small down pointing chevron arrow a couple of times to change that number to '2' Then left click on 'OK' button

See image below to see the steps.

Now you need to exit Thunderbird. Click on 'Menu app icon' (it has 3 horizontal lines) click on 'File' click on 'Exit'

It is important that Thunderbird shuts down because this will force all the various connections between Thunderbird and the server - (where all the emails are stored) to close. It might be a good idea to close down the computer whether it be a desktop or laptop or tablet to ensure nothing is connecting to the internet.

Do you ever look at your email using a different device ? Perhaps, you also use a phone or maybe you have another laptop or tablet which you use to see emails. Those devices may be hogging cached connections as well. This means you need to properly close down those other devices. Do not just put them in any type of sleep or hibernation mode. Turn them off.

You now need to wait for a while whilst the server catches up and realises all the connections for all your folders in account have been shut down. So the server can reset everything. I would go and have a cup of tea.

After a good half an hour, restart Thunderbird.

If you find the images in the above comments are too small: In this forum question a single left click on an image will make the image significantly larger.