Sometimes an email on server with bad headers or something amiss can cause this to occur. Sometimes it is the webmail Junk/Spam folder. Logon to webmail account via browser. Empty the 'Spam/Junk' folder. In Thunderbird retry the 'Get Messages' If still no joy: In webmail account, look for the oldest email that has not downloaded. Read and move to another folder. In Thunderbird retry the 'Get Messages' Please confirm if you keep a copy of all incoming emails on the server. Report back on results.