Hello,
the private .ics url is generated by using the uniqid function, but the documentation says:

This function does not generate cryptographically secure values, and should not be used for cryptographic purposes. If you need a cryptographically secure value, consider using random_int(), random_bytes(), or openssl_random_pseudo_bytes() instead.

This function does not guarantee uniqueness of return value. Since most systems adjust system clock by NTP or like, system time is changed constantly. Therefore, it is possible that this function does not return unique ID for the process/thread. Use more_entropy to increase likelihood of uniqueness.

It would be great, in order to avoid generating guessable secret urls, to use something better, like random_compat, that is already provided by DokuWiki.