I am having a strange issue with Magento 2 sign in admin panel.

It's been working fine but since yesterday the sign in just doesn't work. It shows the panel to enter user and pass but just 'refreshes' on entry - although it does put an index key in the url.

No error message.

Anyone experienced similar or can assist in troubleshooting?

Frontend is working OK (except checkout module).

Additional info: just before we had the issue the Full Page Cache remained invalidated, even when we refreshed. Any edits at FTP level (FileZilla) are not going through giving 'error: failed'. We are on AWS EC2 server instance.

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