Apologies for the trouble @julian7. We’ve been working through this issue over the past couple days and think we’ve found a workaround (albeit a tedious one).
The root of the issue is based on something iOS (& other operating systems) refer to as File Type Association. When any app is installed, the app tells iOS what file types it ‘owns’ and which file types it can ‘open.’ There’s a subtle, but important difference that is causing the issue you’re seeing.
The SketchUp Viewer app registers with iOS and appropriately tells iOS that it ‘owns’ the SKP format. Problem #1 is – so do other apps (not something we have control over, unfortunately). Problem #2 is – iOS offers ‘ownership’ up to whichever app claims it first.
What this means is that if you install a different app that claims ownership of the SKP format, prior to then subsequently installing the SUV app, we’re unable to wrestle ownership of SKP away from that other app (which the Files app apparently needs in order to allow you to open a SKP file from on Device, Dropbox, etc.).
After some trial & error we discovered a workaround, that we’ll admit is a bit lame, but so far is the best we have to offer. Begin by identifying other installed apps that you think might be claiming ownership of the SKP format, and systematically uninstall them, checking to see if the SUV app is then able to open SKP files via the Files app. For example, we followed this process on a test device & discovered that Evernote is one culprit (if you find others, please let us know). Once the offending app is uninstalled, SUV can claim ownership of the SKP format & retain ownership of the format even after the offending app is reinstalled. Meaning, it should then be safe for you to then reinstall whatever apps you had to uninstall, and the format issue should stay fixed. We’ll continue to explore this issue and look for ways to resolve it in a more straight forward way. In the mean time, I hope this info is helpful and that the workaround is effective.