Redmine

This should not happen with default Redmine.

The error you are receiving is a generic issue validation for when you try to update an issue and set either its due date or its start date to an invalid value (i.e. the start date after the due date). However, the start date is only automatically set on newly created issues based in the global setting in Administration -> Settings -> Issues -> Use current date as start date for new issues. For existing issues, Redmine does not automatically set a start date. For existing issues or when not changing either the due date or the start date, this is also not checked. Note that the same validations are also applied when updating an issue by other means, e.g. via your web browser or the API.

Thus, what may happen here is that something tries to set an invalid start date and/or due date. This could be because of a customization in your Redmine or because you are trying to set an explicit date from your mail (e.g. in the mail body or via a mail handler option). In any case, the cause of this issue is likely within your configuration and you should fix this.

Without further details, it is however difficult to actually pinpoint the root cause of this. To be able to further look into this, we need more details about your specific issue and the mail as well as your issue settings. A full log entry from your log/production.log is also helpful.