SSCBrian wrote:So far, I've not had any issues with ISOs on 3.1.0.
Likewise, and I've been using them a lot, all through the beta phase. And of course the guest additions come packaged as an ISO.
I agree with Sasquatch: most likely there is a incorrect field in the ISO header, and this new version of VBox detects it.
@stefanp: if for some reason you can't or don't want to download a replacement for this ISO, then perhaps you could repair the situation by using an ISO-tool to unpack and then repack this ISO, assuming of course that this tool is more forgiving than VBox.