I get the same trying to render to a PNG sequence
From a frame timeline using File > Export > Render video to png sequence I get the "Program Error"
From a video timeline using Render video in the timeline menu the PNG option in Image sequence is greyed out
(CC2018 on Windows 10 v1709)
Unsure if this bug has been reported before or not, but with the CC 2018 upgrade I've noticed that the settings to render animations with transparent backgrounds in Photoshop are grayed out and you can no longer select them...so it's now pretty much impossible to render videos or image sequences with transparency. I used to do this all the time, and have no idea why this is suddenly the case.
So for example, in the past if I wanted to render an animation in the timeline with a transparent background I could go File>Export>Render Video and then in the settings select "Photoshop Image Sequence" set the file format to PNG from the drop down menu, then set the alpha channel to "straight-unmatted" under the Render Options, and I'd be good to go. But with the update to CC 2018, for some reason the PNG file format and Alpha Channel settings are grayed out, so you can't actually select them. It makes no sense.
Similarly, I'm also unable to export animations in the timeline as MP4's with transparent backgrounds, because, like with the image sequence formats, the option to change the alpha channel settings under Render Options is grayed out so you can't select it. This was actually something I noticed recently in CC 2017 as well, but I was able to get around the problem by exporting my animations as PNG Sequences instead, which as I just mentioned, is no longer possible for some reason. This problem is SUPER weird to me, because I can render transparent MP4's in After Effects perfectly fine, I only encounter the problem in Photoshop and they both use Adobe Media Encoder for this function.
I know a few other folks who are having the exact same problem on both Mac and Windows machines so I'm pretty sure it's a bug on Adobe's end. This is a pretty big hindrance and I would really appreciate it if someone from Adobe could take a look into this. (But if anyone else experienced these problems and was able to fix them somehow, I'm open to your suggestions.) If it matters I'm running Windows 10 Build 15063. Thanks!
I am happy to announce that this issue should be resolved in the release of Photoshop that just went live, 19.1. Please open your Creative Cloud desktop app, update Photoshop and try it out!
Adobe Blog: Selections Improvements, Advanced Support for Windows High-Density Monitors and More
What's new: https://helpx.adobe.com/photoshop/using/whats-new.html
What's fixed: https://helpx.adobe.com/photoshop/kb/fixed-issues.html
Instructions for updating: https://helpx.adobe.com/creative-cloud/help/update-app.html