OP5 Monitor - How to fix Report generation fails: "There was an error rendering the page", Failed to parse time string (...) at position 0 (2): Unexpected character

This PHP error indicates that a time string indicating a date and time couldn't be parsed. A common reason for this in OP5 Monitor is that the date_format setting in naemon.cfg has been changed.

How it should look

The default value of the date_format setting in naemon.cfg is:

date_format=iso8601

We strongly recommend that you keep this default setting.

What may cause problems

If you set this value to another one of the listed options, for example "euro", your timestamps will now look like this: 23/07/2019 16:00:00. Since there aren't 23 months in a year, we can tell that this means DD/MM/YYYY, right?

The problem is when you pass a string like this to PHP, it may interpret this as an "American" MM/DD/YYYY type string, which doesn't make any sense -- and you get an error:

modules/reports/controllers/base_reports.php [281]:
DateTime::__construct(): Failed to parse time string (23/07/2019 16:00:00) at position 0 (2): Unexpected character

Summary

Do not change the date_format setting in naemon.cfg unless you have very good reason to do so, and know that it may break reports.