For some silly reason, exporting files over webUSB from a Remarkable 2 had a 30 second timeout... which can be longer than the tablet needs to process and convert it's larger, handwritten files on device.
This is fixable in a quick patch.
You'll need to edit the /usr/share/remarkable/webui/assets/index.js file. It's minimised js, so you'll want to use nano or some other fast edior.
Using Ctrl+W, search for "download/" in the file and then do the following edit:
Important
Increase the 3e4 timeout in fetch(`/download/${e}/${t}`,{method:"get",credentials:"same-origin",signal:AbortSignal.timeout(3e4)}) to something like 3e5