Whenever I enter the world, the calendar button is invisible. However, if I hover the cursor over it, it shows the tip. It is also possible to click the button, but still it's invisible. The only way to make it appear is to move the button.

This will be caused by another addon. Please try disabling all your other addons until the issue goes away. Let me know what addon was the culprit.

I have disabled all addons except SexyMap and the issue didn't go away. I also tried to delete folder Cache and it didn't help.

Then the only solution left is that you've changed the setting for the calendar button to show on hover, and somehow not realized you've done this.

No response, closing.