Today in the Thunderbird calendar multiple light blue & dark blue columns top to bottom have appeared side by side extending into the first week of September

For no apparent reason the calendar display has been filled with multiple columns of light and dark blue, side by side and extending top to bottom. This pattern starts in The August 31st column and continues up to and including September 7th. After that the display goes back to normal. Does anyone know why this has happened and how to fix it? Thank you.