[TYPO3-english] BE Users cannot login suddenly on 4.5 LTS site

[Tony Lush]

On a 4.5 LTS site BE users suddenly cannot log in with their correct credentials. I could use the Install Tool to create a new admin user, but that user also could not log in.

The website FE functions normally.

All other 4.5 and 6.2 LTS sites on the same server do not have the issue.

Any suggestions?

Kind regards -- Tony