Might be related to bug #69122 "strtotime and DateTime fail to parse certain strings with fractional seconds".

Encountered this one today too, while parsing actual timestamps received from another party.

Following paste shows it still works up to 8 digits, and fails at 9 (without a timezone suffix, which makes the error message even more cryptic):
https://3v4l.org/pMvig

It also shows that the fraction isn't handled correctly with 8 digits (23:59:59.1000000)

Unfortunately I don't have the resources available to verify with a build of php-src right now.

But, I think, maybe this is related to the max length of 8 at line 1245 of https://github.com/php/php-src/blob/master/ext/date/lib/parse_date.re

time->us = timelib_get_frac_nr(&ptr, 8);

Maybe if that was increased to at least 9 (nanosecond) precision. Other systems are doing nanosecond precision and interoperability problems will occur with this limitation.

Thank you for your bug report. This issue has already been fixed
in the latest released version of PHP, which you can download at
http://www.php.net/downloads.php

Both variants of the original comment parse in PHP 8.1, although PHP does not have a high resolution than microseconds:

<?php
var_dump(new \DateTime("2016-01-01T01:02:03.123456789-01:02"));
var_dump(new \DateTime("2016-01-01T01:02:03.12345678-01:02"));
?>

Standard input code:2:
class DateTime#1 (3) {
  public $date =>
  string(26) "2016-01-01 01:02:03.123456"
  public $timezone_type =>
  int(1)
  public $timezone =>
  string(6) "-01:02"
}
Standard input code:3:
class DateTime#1 (3) {
  public $date =>
  string(26) "2016-01-01 01:02:03.123456"
  public $timezone_type =>
  int(1)
  public $timezone =>
  string(6) "-01:02"
}