Excel for the web is no longer able to load files.

Hello,

My friend and I share multiple excel files that we use regularly.
So far that worked like a charm!
We've done so for years and we never really had any problems.
But since this week, the larger files won't open anymore. Even files that we don't update anymore and always opened in Excel for the web, won't open anymore.
Has anything changed?
I've tried different browsers, clearing the browser cache, but it isn't working.
Does anyone have any other ideas?

Thanks in advance!