A Microsoft file hosting and synchronization service.
"we're sorry, someone updated the server copy and it's not possible to upload your changes now." even for every old files that I haven't touched for years. You're then asked to "keep my version" or "keep server version". They both appear to be the same. The error message, I think, is generated by OneDrive.
It happens in all Office 2016 applications.
It's the same issue 光锐杨 has reported.