Please send a screenshot of the message. It would be good to have the reason.

Error message (in Swedish): "Det gick inte att ansluta till imap.privat.bahnhof.se. Försök igen senare. Om problemet kvarstår, kan du ha överskridit maximala antalet anslutningar till servern. Du kan minska antal cachade anslutningar i IMAP-serverinställningarna" It makes no difference if I set the number of cached connections to 1 or 15.

re error : "Unable to connect to imap.privat.bahnhof.se. Please try again later. If the problem persists, you may have exceeded the maximum number of connections to the server. You can reduce the number of cached connections in the IMAP server settings"

The server will have a maximum number of connections that it will allow. You cannot change that situation. Connections to server are not just Thunderbird, so if you have other computers/ tablets/ phones that are accessing the same email address/account on server then the connections mean from all those devices - cummulative count.

The most common problem that can cause this issue is if the user also uses a phone to access emails. The phone is usually not switched off completely and that means it keeps checking for emails in the background.

I did come across a user who had more than one computer accessing the account - one at work and one at home. The one at work was still connecting to the server and using up connections, so the user has a problem connecting on the home computer.

Another user discovered they had Outlook installed - they had used it a long time ago, but after I asked them to check it out they discovered it was still connecting to server in the background. They deleted the mail account from Outlook and made sure it was disabled by default when computer was started.

How to fix.: You have to force those connections to close on all devices that can connect to server.

Switch off the phone to force the email to close connections. Also see if there is an auto setting forcing it to check for mail and disable it.

For all computers/tablets etc accessing server via Thunderbird. Set the cached connections to 1 or 2. By default, Thunderbird set this to 5 which would be ok for many servers and if user is not creating a lot of other connections using other devices. Then exit Thunderbird.

Check all devices - do you have any other email clients installed on computer/tablet etc which are running in the background and you are not aware of it. You need to remove accounts from those programs and stop them working in background.

Leave everything switched off for about half an hour to ensure the server has decided all connections have been properly disconnected. Have a cup of tea.

then restart Thunderbird.