Problem
When I want to open an (for instance Word) attachment in Evolution e-mail (running in Flatpak) of which OnlyOffice is the default application (also running in Flatpak). OnlyOffice is started, but it does not seem to have access rights to that attachment.
Possible cause
When I open a PDF attachment in Evolution, the PDF document reader is started, so the problem only seems to occur when opening OnlyOffice attachments in Evolution.
OnlyOffice shows the behavior (opening a blank document) when OnlyOffice does not has access rights to a document. So I played with Flatseal (allowing OnlyOffice more access rights (system, home, etc ) but that did not solve the problem.