Microsoft Internet Explorer and Microsoft Edge
This happens when MetaShare cannot find the cookie set by "login.microsoftonline.com".
The cookie is required in order to use functions such as upload, delete and check in, functions that send requests directly to SharePoint.
This can happen if "app.metashare.com" and "login.microsoftonline.com" are added to different browser security zones.
If "app.metashare.com" or "login.microsoftonline.com" are explicitly added to the "Local Intranet" or the "Trusted sites" security zones they should both be in the same zone or one in the "Local Intranet" zone and the other in the "Trusted sites" zone.