How are evolution images attached to email reports? Ours show missing images

Hi, I’ve created a custom report on Matomo 5 which creates an evolution with a filter on one page.
When I download the email report where this custom report is attached to, it looks like this:
But sending it, the image is always missing:

Not sure if this is relevant, but we have a custom config with 2 versions of Matomo running on the same database server.
One is used for tracking and has a config which blocks logins by using maintenance mode.
The other does the heavy lifting and processing but can only be accessed through a tunnel. The second one is the one sending the emails.

Just sent the report to a different email address as I can’t have a look at the full source in Outlook.
The image is attached in Thunderbird (while it’s completely missing in Outlook).

The image is added to the email body like this:

<img alt=3D"" src=3D"cid:CustomReports_getCustomReport_idCustom=
Report--15" height=3D"200" width=3D"700" margin=3D"0 auto">

And it’s attached to the email like this:

Content-Type: image/png; name="=?utf-8?Q?Author_guidelines_evolution_-_week_May_13?=
Content-Transfer-Encoding: base64
Content-ID: <=?us-ascii?Q?CustomReports=5FgetCustomReport=5FidCustomRepor?=
Content-Disposition: inline; filename="=?utf-8?Q?Author_guidelines_evolution_-_week_May_13?=



I don’t know enough about emails to tell where the problem sits.