stuck with the same problem, i found the solution on slice export, when you choose "export all" it open Files App, if you want to put your files, don't go inside your folder. outside of the folder, press select on top right, select /check box your folder, and then press "Open" on top right..
This happened to me after trying affinity designer ipad for one week with no problem, suddenly the export won't work. when i try to export artboard(s), it opened the Files App but it looks like Files App got wrong command, no "ok" button (see video). now I'm stuck with export button (in dropdown menu, nit from slice) and it's so annoying because i have to export multiple artboard one by one. i tried reinstalling the Files app but it doesn't get normal, haven't tried reinstalling the affinity designer app. please help. 9A7525E6-43A2-4B66-A202-C8D4E58F49DF.MP4