I got the same error on a Redmi 4A, however when presented with the list
of apps I could upload the file to, another option for me was Google
Drive. Uploading to that worked for me and integrated well into my
workflow.
I hope that helps. I just started using the app two days ago, so I
don't know if this is a new problem or not. There's a forum for the
tagtime system on the Beeminder website where you might find additional
help:
http://forum.beeminder.com/c/tagtime
-Dave