icloud webmail became useless!

Hi to all,

For a few days now, without any changes on my part, I have been experiencing on several computers (macOS and Windows) and with different browsers (latest Firefox and Edge) that new mail in my webmail inbox cannot be marked as read by the system, which is notified by a system window that appears shortly after opening the mail. Furthermore, deletion, i.e. moving items to deleted items, also encounters similar errors, but an error message is returned when the deleted items are emptied.

Using the same icloud account from an iphone device, I can manage it flawlessly with the default mail application.

I've tried everything I can, including clearing the browsers' full cache and cookies.

Please help!

Gabor