Same here, I just opened the drill files from the project and GerbView shows them fine.
It looks like the manufacturer has old CAM software or something and can’t interpret slots.
But DirtyPCBs isn’t even a manufacturer. Here’s a quote from their website:
Readme.txt & special instructions
Get the hell out of here. We are an automated conduit to cheap local Chinese fabs. We don’t read squat, and at these prices you’d be insane to think we do! Nobody at the board house even speaks English! You need a much more expensive board house if you need this level of service.
I would just change your “manufacturer” and order directly from some manufacturer. Most of them can make slots, even the cheap ones.
Try a different paneliser, like KiKit. That GerberTools paneliser is a nightmare because it’s written in C# (worse than Java for obfuscated error messages) and I never got any version later than 2017 to work with Mono on Linux.