I did a little investigation, please file a ticket with Support, this seems like a bug:
Up until Mendix 8.3 the DojoX library was included (for example, the retracted 8.3 had: Program Files\Mendix\8.3.0.61600\runtime\mxclientsystem\dojox and Program Files\Mendix\8.3.0.61600\modeler\tools\node\dojox)
8.4 seems to have this library missing, which will most likely break a lot of the older widgets.
The folders mentioned by Jelte are indeed missing. We’ve received the advice from support to copy the dojox folder in runtime/mxclientsystem from 8.3 to 8.4. This didn’t actually fix anything as you also have to copy the other dojox folder (modelers/tools/node/). After that you can build… but not via cloud deployment - as the build process doesn’t use your client I guess.
By building a deployment package locally and uploading it manually you can deploy like you’d want to. I’ve just successfully deployed a project with 8.4 to a node like this after getting these errors on six widgets.
N.b. Mendix is working on a fix.