Hey there @vdauv - sorry to hear about this and apologies for the tardy response.
Can you please let me know why you're not able to open this file (or files) online? Are you getting an error? If so, could you share a screenshot so we can have a visual too? While we're at it, have you tried any other browsers other than the one you normally use?
As you describe the other users are not able to edit, did you by any chance shared a link with them and not a folder directly? Anyone with the link can view the contents, but can’t edit them. If you shared the folder normally and they still get the options to download only instead of opening or editing, it means that they don't have the appropriate software (i.e. Microsoft's Excel in this case) installed on their devices and also it could mean that the files haven't synced yet or that they don't have the desktop application installed and running.
I hope this helps point you to the right direction and please keep me posted on your findings. Thank you in advance for your reply and feel free to include any information you think might help our investigation. Until we speak again, have a lovely day!