Just to confirm, is this the URL you’re using? It comes up fine for me. Might be a Flockstr bug?

https://www.flockstr.com/calendar/naddr1qqjrsd6xggcnxsek95m5vvp4956yv32y95urjv2x95cyywzpxeqnz3zp8qeyxqgdwaehxw309ahx7uewd3hkcq3qc0nfstrlj0jy8kvl953v84hudwnpgad0zx709z0ey7nmjp0llegsxpqqqp7tg0ereug