After testing, I was unable to reproducer this error on the current Discourse release.
Discourse - 2.9.0.beta2
Discourse Calendar - v0.2
Microsoft Windows 10 Pro - Version 10.0.18363 Build 18363
Google Chrome - Version 99.0.4844.74
Navigating to [discourse.example.com]/upcoming-events appears to work correctly and displays the calendar as intended.
