If you just updated Thunderbird, the most obvious reason is that something went wrong during the update, such as some files not being replaced properly, maybe because some antivirus software prevented one or more files from updating. Try this: # Uninstall Thunderbird (your user data and settings including mail should be preserved) # Remove Thunderbird’s program folder if it still exists #* If you see a warning about (a) file(s) not being removed, that may indicate something # Install Thunderbird from [https://www.mozilla.org/nl/thunderbird/ here] Does the error still show up?